This fixes an error in production builds of the client in Safari 10:

```
SyntaxError: Cannot declare a let variable twice: 't'.
```

Fixes #2664

 - Remove `seamless` attribute which has not been supported by major
   browsers for years [1]
 - Remove the `name="hyp_sidebar_frame"` attribute which is not
   referenced anywhere else in the code. I did find a couple of
   references to it in Google, but only in scripts written by Hypothesis
   team members which are not run regularly.

[1] https://caniuse.com/iframe-seamless

Update Babel compilation targets to match the browsers we currently support,
rather than transpiling everything down to ES5.

This reduces the size of our JS bundles and improves performance by using native
implementations of ES2015+ language features. It should also avoid
occassional issues in development caused by interactions of Babel plugins that
compile native features to older syntax (eg. `transform-async-to-promises`).

The client's boot script has a different configuration than the rest of the
client because it needs to be able to run in older browsers in order to show a
warning when the browser is not supported.

The production bundle size changes resulting from this are:

Sidebar bundle: 398 KB => 329 KB
Annotator bundle: 219 KB => 169 KB

Native (non-transpiled) arrow functions cannot be used with the `new` operator.
This commit fixes a couple of cases where that was happening.

The `Host` class is never constructed directly and only has one
subclass, `Sidebar`. There wasn't a clear separation of responsibilities between
`Host` and `Sidebar`. This commit therefore moves the functionality of `Host` into
`Sidebar` and merges the tests.

Restructure the way that buckets are generated and represented. Simplify
what `BucketBar` needs to do with the buckets. Fix a bug in which
anchors can be occasionally included in multiple buckets.

Fixes #397
Fixes #2618

Code in the `boot/` module needs to be compiled to work in older
browsers than code shared between the annotator and sidebar applications
in `src/shared`. To ensure this it helps to avoid dependencies from
code in `src/boot` on code in `src/shared`, but the converse is OK.

This commit moves the `jsonConfigsFrom` function from `shared/settings`
to `boot/parse-json-config` so that it is compiled with the same
settings as the rest of the code in the `boot/` module and to reduce the
chances of someone accidentally introducing ES6+ features into this code
in future and forgetting about older browsers.

The function was also renamed to make it more obvious that it parses
JSON config, and thus may fail, rather than returning a JSON string.

The code in `polyfills/index.js` which determines which polyfills are
needed are compiled as part of the boot script and not used by other
parts of the application.

Because this code is compiled as part of the boot script it needs to be
compiled to parse in older browsers, even those which don't support the client
(eg. IE 10/11). Putting the `polyfills/` dir in side `src/boot` means
that it will inherit any custom Babel configuration for the `src/boot`
directory.

`document.currentScript` is supported by all browsers that we support.
If it returns `null` this indicates that it was called in the wrong
context and so fail loudly to make this obvious.

Remove workarounds for IE that are no longer needed since IE 11 is not
supported any more.

 - Remove optional and deprecated arguments to `document.createNodeIterator`

 - Update references to IE 11 for checks/code which is still applicable
   to other browsers

 - Remove `window.msCrypto` fallback for `window.crypto`

 - Remove unused `isIE11` helper

 - Remove IE 11 reference in `polyfills/index.js`. We don't actually
   need the check because the browser check in `boot/index.js` will fail
   if `Promise` is missing, but I left it in because that knowledge is
   contained in a separate part of the code and
   `shared/polyfills/index.js` is currently written as if it may be used
   in a pre-ES2015 environment.

This banner is no longer needed because the client does not support IE
11 and aborts startup in that browser.

Bumps [preact](https://github.com/preactjs/preact) from 10.5.4 to 10.5.5.
- [Release notes](https://github.com/preactjs/preact/releases)
- [Commits](https://github.com/preactjs/preact/compare/10.5.4...10.5.5)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Bumps [postcss](https://github.com/postcss/postcss) from 8.1.1 to 8.1.2.
- [Release notes](https://github.com/postcss/postcss/releases)
- [Changelog](https://github.com/postcss/postcss/blob/master/CHANGELOG.md)
- [Commits](https://github.com/postcss/postcss/compare/8.1.1...8.1.2)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Bumps [terser](https://github.com/terser/terser) from 5.3.5 to 5.3.7.
- [Release notes](https://github.com/terser/terser/releases)
- [Changelog](https://github.com/terser/terser/blob/master/CHANGELOG.md)
- [Commits](https://github.com/terser/terser/compare/v5.3.5...v5.3.7)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Bumps [mocha](https://github.com/mochajs/mocha) from 8.1.3 to 8.2.0.
- [Release notes](https://github.com/mochajs/mocha/releases)
- [Changelog](https://github.com/mochajs/mocha/blob/master/CHANGELOG.md)
- [Commits](https://github.com/mochajs/mocha/compare/v8.1.3...v8.2.0)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Bumps [@babel/core](https://github.com/babel/babel/tree/HEAD/packages/babel-core) from 7.11.6 to 7.12.3.
- [Release notes](https://github.com/babel/babel/releases)
- [Changelog](https://github.com/babel/babel/blob/main/CHANGELOG.md)
- [Commits](https://github.com/babel/babel/commits/v7.12.3/packages/babel-core)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Bumps [@babel/preset-react](https://github.com/babel/babel/tree/HEAD/packages/babel-preset-react) from 7.10.4 to 7.12.1.
- [Release notes](https://github.com/babel/babel/releases)
- [Changelog](https://github.com/babel/babel/blob/main/CHANGELOG.md)
- [Commits](https://github.com/babel/babel/commits/v7.12.1/packages/babel-preset-react)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Bumps [@babel/preset-env](https://github.com/babel/babel/tree/HEAD/packages/babel-preset-env) from 7.11.5 to 7.12.1.
- [Release notes](https://github.com/babel/babel/releases)
- [Changelog](https://github.com/babel/babel/blob/main/CHANGELOG.md)
- [Commits](https://github.com/babel/babel/commits/v7.12.1/packages/babel-preset-env)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Bumps [eslint-plugin-react](https://github.com/yannickcr/eslint-plugin-react) from 7.21.4 to 7.21.5.
- [Release notes](https://github.com/yannickcr/eslint-plugin-react/releases)
- [Changelog](https://github.com/yannickcr/eslint-plugin-react/blob/master/CHANGELOG.md)
- [Commits](https://github.com/yannickcr/eslint-plugin-react/compare/v7.21.4...v7.21.5)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Bumps [aws-sdk](https://github.com/aws/aws-sdk-js) from 2.771.0 to 2.774.0.
- [Release notes](https://github.com/aws/aws-sdk-js/releases)
- [Changelog](https://github.com/aws/aws-sdk-js/blob/master/CHANGELOG.md)
- [Commits](https://github.com/aws/aws-sdk-js/compare/v2.771.0...v2.774.0)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Add comments to explain where the `#mainContainer` selector come from,
when the warning banner size may change and what happens in older
browsers.

Divide up `toast-message` styling for resuability. Use its base
styles as a foundation for a full-width "banner" pattern.

This icon is an MIT-licensed SVG sourced from https://feathericons.com/

 - Add `documentHasText` helper in `anchoring/pdf.js` which checks if
   the document appears to have selectable text

 - Add logic in `plugin/pdf.js` which uses the `documentHasText` helper
   to check whether the PDF has selectable text and display a banner
   above the PDF viewer if not

Fixes https://github.com/hypothesis/product-backlog/issues/1081

Following #2645 jQuery is no longer used by the Hypothesis client.

Remove jQuery dependency from BucketBar, its tests
and other relevant modules' tests involving BucketBar.

Opportunistically add some type-checking, commenting and
other small improvements but leave general structure as-is.

Increase test coverage.

Previously, the `buildBuckets` function returned an
object with two properties: `buckets` and `index`.

Change to return an Array of `Bucket`s and type-define
what a `Bucket` is—`Bucket` objects now include their
`position` information, obviating the need for an
`index` Array that could get out of sync.

f96af43d added support for PDF.js >= v2.5.207 by changing an event
listener to use PDF.js's internal event bus rather than DOM events.
However there was another DOM event listener for the `pagesloaded` event
in `src/annotator/anchoring/pdf.js` that should also have been updated but was
overlooked. This didn't cause a problem in testing with the dev server because
the test documents load quickly enough that they are already loaded by the time
the client's anchoring logic ran.

This commit updates the way that the client listens for events from
PDF.js to use the event bus where available and only fall back to the
DOM in versions of PDF.js that don't support it.

 - Use PDF.js's event bus to listen for `documentload`/`documentloaded`
   and `pagesloaded` events

 - Add a fallback method to wait for event bus to become available in
   versions of PDF.js which support the eventBus but don't have the
   `initializedPromise` API

 - Improve the documentation around which versions of PDF.js support
   different event types and event dispatch methods

 - Add tests to cover the behavior from different releases of PDF.js

For an overview of the different versions of PDF.js that the client
needs to support, see https://github.com/hypothesis/client/issues/2643.

`TagEditor` tested if the browser was IE 11 to determine whether to
check the `inputType` property of `InputEvent`. However it is not only
IE 11 that did not support this property.

From reading the comments in https://github.com/hypothesis/client/pull/1558 it
appears that the `inputType` checks was originally added to resolve an
issue with accepting suggestions by pressing Enter in an earlier version
of `TagEditor` that used the `<datalist>` element. As far as I can tell
this is no longer applicable. `<datalist>` was replaced by a custom UI
and accepting suggestions by pressing Enter works without this logic.
Therefore this commit removes it entirely.

The sidebar application failed to load in Safari 10 due to the ES 2018
polyfill bundle, which contains a polyfill for
`Promise.prototype.finally`, not being loaded in that browser.

The issue was that 'es2018' was not listed in the polyfill bundles to
build in `gulpfile.js` or in the list of bundles required by the sidebar
app in `boot.js`.

This commit fixes these issues and makes the issue less likely to occur
in future by:

 - Removing the need to manually list each polyfill bundle in
   `gulpfile.js`. Instead the `src/shared/polyfills` dir is listed and
   every non-`index.js` file is used as the root of a bundle.
 - Factoring out the polyfills common to the sidebar and annotator app
   into a variable and including "es2018" there

Element.closest is natively supported [1] by our minimum browser targets.

[1] https://developer.mozilla.org/en-US/docs/Web/API/Element/closest#Browser_compatibility

This API is available in Edge Legacy but with a quirk that arguments
that are optional in the spec are required in this browser.

This fixes an issue that the client would incorrectly abort startup in
Edge Legacy with a warning that the browser is unsupported.

This polyfilled the optional-ness of the `filter` and deprecated
`expandEntityRefrences` arguments of `document.createNodeIterator` in IE
11, which is no longer supported.

This was only needed for IE 11, which is no longer supported.

This was only needed for IE 11, which is no longer supported.

This was only needed for IE 11, which is no longer supported.

Run a series of feature tests at the very start of the boot process and
abort with a console warning if they fail.

We display a console warning rather than a louder warning to avoid being
disruptive on sites maintained by publishers which embed Hypothesis and
which still want to support older browsers. On those sites the
Hypothesis annotation feature will simply be missing.

The sidebar app failed to start in Edge Legacy due to `element.dir
= "auto` assignments causing an exception during Preact component
rendering. We already had a fix for this that applied to IE 11, so the
fix for this problem is to change the check to test for the bug
specifically rather than testing the user agent.

Bumps [sass](https://github.com/sass/dart-sass) from 1.26.12 to 1.27.0.
- [Release notes](https://github.com/sass/dart-sass/releases)
- [Changelog](https://github.com/sass/dart-sass/blob/master/CHANGELOG.md)
- [Commits](https://github.com/sass/dart-sass/compare/1.26.12...1.27.0)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>