Extract styling for indicator buttons in bucket bar, organize and
comment it

Create a preact component `Buckets`, and sub-components `BucketButton`
and `NavigationBucketButton`. This refactors buckets to add some
HTML semantics and accessibility.

Apply clean-theme class to "root" elements of sidebar
and annotator. Remove clean-theme logic from sidebar components, and
instead use selector in SASS modules to apply relevant styles.

Fixes https://github.com/hypothesis/client/issues/2617

Bumps [postcss-url](https://github.com/postcss/postcss-url) from 8.0.0 to 10.0.0.
- [Release notes](https://github.com/postcss/postcss-url/releases)
- [Changelog](https://github.com/postcss/postcss-url/blob/master/CHANGELOG.md)
- [Commits](https://github.com/postcss/postcss-url/compare/8.0.0...10.0.0)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Add a test for the `Element.prepend()` method so that startup fails more
gracefully in older, unsupported versions of Edge Legacy (<= 16). This
API was chosen as a representative test because it's Chrome/Edge Legacy
support closely matches our minimum versions.

The same change was made for the LMS app in https://github.com/hypothesis/lms/pull/2209.

Bumps [commander](https://github.com/tj/commander.js) from 6.1.0 to 6.2.0.
- [Release notes](https://github.com/tj/commander.js/releases)
- [Changelog](https://github.com/tj/commander.js/blob/master/CHANGELOG.md)
- [Commits](https://github.com/tj/commander.js/compare/v6.1.0...v6.2.0)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Bumps [puppeteer](https://github.com/puppeteer/puppeteer) from 5.3.1 to 5.4.0.
- [Release notes](https://github.com/puppeteer/puppeteer/releases)
- [Commits](https://github.com/puppeteer/puppeteer/compare/v5.3.1...v5.4.0)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Bumps [terser](https://github.com/terser/terser) from 5.3.7 to 5.3.8.
- [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/v5.3.7...v5.3.8)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Bumps [postcss](https://github.com/postcss/postcss) from 8.1.2 to 8.1.4.
- [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/8.1.2...8.1.4)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Bumps [eslint-plugin-jsx-a11y](https://github.com/evcohen/eslint-plugin-jsx-a11y) from 6.3.1 to 6.4.1.
- [Release notes](https://github.com/evcohen/eslint-plugin-jsx-a11y/releases)
- [Changelog](https://github.com/jsx-eslint/eslint-plugin-jsx-a11y/blob/master/CHANGELOG.md)
- [Commits](https://github.com/evcohen/eslint-plugin-jsx-a11y/compare/v6.3.1...v6.4.1)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Bumps [@sentry/browser](https://github.com/getsentry/sentry-javascript) from 5.27.0 to 5.27.1.
- [Release notes](https://github.com/getsentry/sentry-javascript/releases)
- [Changelog](https://github.com/getsentry/sentry-javascript/blob/master/CHANGELOG.md)
- [Commits](https://github.com/getsentry/sentry-javascript/compare/5.27.0...5.27.1)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Bumps [typescript](https://github.com/Microsoft/TypeScript) from 4.0.3 to 4.0.5.
- [Release notes](https://github.com/Microsoft/TypeScript/releases)
- [Commits](https://github.com/Microsoft/TypeScript/compare/v4.0.3...v4.0.5)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Bumps [aws-sdk](https://github.com/aws/aws-sdk-js) from 2.776.0 to 2.779.0.
- [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/v2.776.0...v2.779.0)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Bumps [eslint](https://github.com/eslint/eslint) from 7.11.0 to 7.12.1.
- [Release notes](https://github.com/eslint/eslint/releases)
- [Changelog](https://github.com/eslint/eslint/blob/master/CHANGELOG.md)
- [Commits](https://github.com/eslint/eslint/compare/v7.11.0...v7.12.1)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Attempt to fix a prod deployment failure when initial build step happens
in a different workspace directory than the prod deploy step.

This typically happens when two or more builds are in progress concurrently
and then all of the earlier builds finish before the last one is
deployed. In this scenario the initial build step happens in a directory
such as `master@2` whereas the prod deploy step will happen in a
directory named `master`. The prod deploy step assumed that the
workspace was still `master@2` and git commands would fail because the
directory no longer existed.

Remove polyfills for features that are supported by all of our target
browsers:

 - `fetch` (https://caniuse.com/fetch)
 - The `URL` constructor (https://caniuse.com/url)
 - `String.prototype.normalize` (https://caniuse.com/mdn-javascript_builtins_string_normalize)

The transform-async-to-promises Babel plugin has been replaced by native
async/await support in most of the code, except for the boot script.

This resolves errors building some indirect dependencies in the most
recent versions of Node.

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.