Make several improvements following PR review.

Implement a new `useStoreProxy` hook that provides access to read and
update the store in UI components. This will replace the existing
`useStore` hook.

The new hook has a more ergonomic API which should also prevent some of the
common mistakes made when using the previous hook.

`useStoreProxy` returns an ES proxy for the store. The UI component can
use the proxy as if it were using the store directly. The proxy however
tracks and caches the results of store selector calls. When the store
state changes the proxy checks whether any of the cached calls would
return different results and if so invalidates the cache and re-renders
the component.

This makes store updates faster in debug builds when a lot of data is
loaded into the client. In this case most sub-trees in the store are
already frozen after an update (since they are objects from the previous
state) and only the new parts need to be frozen.

Replace remaining uses of `store.getState()` in UI components with
selectors. This will avoid unnecessary re-rendering after we change
components to use the new `useStoreProxy` hook for reading from the
store. It also simplifies the tests.

Profiling anchoring of large numbers of annotations in Chrome showed
that there was significant overhead for scheduling and
executing `requestAnimationFrame` callbacks. In the case of `Guest.anchor`, the
callback often did nothing (it called `removeHighlights` with an empty
array).

There isn't a need for this any more AFAICS so just invoke the logic
synchronously, which is much cheaper.

This was recomputing frequently even though the result only changes when
the selected tab changes.

Also convert `selectionState` to use `createSelector` to avoid it
returning a different reference on each call.

Remove unused selector and tidy up reference ordering

Refactor selectors for evaluating `hasAppliedFilter`
to remove cross-module state dependencies.

Eliminate the enormous `threadState` `rootSelector`
and re-implement as multiple selectors. Refactor
`useRootThread` and threading integration tests
to account for these changes.

Refactor the computation of "filter state", that
is, all of the store state that impacts what
constitutes applied filters on annotations.

The `selection` store module has become overlong and
its responsibilities aren't clear. We know we'll be
adding some more filtering capabilities to the app
in the next short while, and that would make
`selection` even more complex and heavy.

Split into two store modules: `selection` and
`filters`.

Temporarily re-implement `rootSelector`s that
are needed for generating thread and filter
state for components.

The name `dir` is arguably ambiguous in this context.

When resolving a `TextRange` to a DOM `Range` there is an edge case
where the `start` or `end` position have an `offset` of `0` and the
element contains no text nodes.

Before this commit `toRange` would throw an `Offset exceeds text length`
error. A more useful behavior though is:

 - For the `start` position, to resolve it to the start of the next text node after
   `this.start.element`

 - For the `end` position, to resolve it to the end of the previous text
   node before `this.end.element`

This commit implements this behavior by first adding a `dir` option to
`TextPosition.resolve` to control what happens when the position's
offset is 0 and the element has no text and then specifying this option
when calling `this.{start, end}.resolve` in `TextRange.toRange`.

Bumps [mustache](https://github.com/janl/mustache.js) from 4.0.1 to 4.1.0.
- [Release notes](https://github.com/janl/mustache.js/releases)
- [Changelog](https://github.com/janl/mustache.js/blob/master/CHANGELOG.md)
- [Commits](https://github.com/janl/mustache.js/compare/v4.0.1...v4.1.0)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Bumps [cross-env](https://github.com/kentcdodds/cross-env) from 7.0.2 to 7.0.3.
- [Release notes](https://github.com/kentcdodds/cross-env/releases)
- [Changelog](https://github.com/kentcdodds/cross-env/blob/master/CHANGELOG.md)
- [Commits](https://github.com/kentcdodds/cross-env/compare/v7.0.2...v7.0.3)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Bumps [@sentry/browser](https://github.com/getsentry/sentry-javascript) from 5.27.6 to 5.29.0.
- [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/5.27.6...5.29.0)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Bumps [postcss](https://github.com/postcss/postcss) from 8.1.10 to 8.1.14.
- [Release notes](https://github.com/postcss/postcss/releases)
- [Changelog](https://github.com/postcss/postcss/blob/main/CHANGELOG.md)
- [Commits](https://github.com/postcss/postcss/compare/8.1.10...8.1.14)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Bumps [core-js](https://github.com/zloirock/core-js) from 3.8.0 to 3.8.1.
- [Release notes](https://github.com/zloirock/core-js/releases)
- [Changelog](https://github.com/zloirock/core-js/blob/master/CHANGELOG.md)
- [Commits](https://github.com/zloirock/core-js/compare/v3.8.0...v3.8.1)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Bumps [sass](https://github.com/sass/dart-sass) from 1.29.0 to 1.30.0.
- [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/1.29.0...1.30.0)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Bumps [aws-sdk](https://github.com/aws/aws-sdk-js) from 2.799.0 to 2.804.0.
- [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/v2.799.0...v2.804.0)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Bumps [eslint](https://github.com/eslint/eslint) from 7.14.0 to 7.15.0.
- [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/v7.14.0...v7.15.0)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Bumps [@octokit/rest](https://github.com/octokit/rest.js) from 18.0.9 to 18.0.12.
- [Release notes](https://github.com/octokit/rest.js/releases)
- [Commits](https://github.com/octokit/rest.js/compare/v18.0.9...v18.0.12)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

On iOS the element must be an `input` (or `textarea`)  and with attribute `'contenteditable'`

Closes #858

Previously, tab selection for direct-linked annotations was happening
before anchoring was complete—that means that orphaned annotations had
not yet been marked as being orphans (`$orphan`). Make sure that
direct-linked tab selection is checked any time the direct-linked
annotation changes.

Also fix a UI confusion in which the "Show All" button would show
the count of all annotations in this (direct-linked) state, which is
confusing because orphans don't count toward "All annotations" (nor do
page notes, FWIW).

Fixes #2686

As indicated by @robertknight `document.styleSheets` is complete only when stylessheets are processed. This can take a while for certain heavy pages.

The alternative approach presented here to find `annotation.css` is warranted to find the url when the code is run.

Closes #2752

 - Add comment to explain purpose of `index.js`
 - Use a template string for readability

This feature is now enabled by default.

Per discussion on the PR, it is preferable to put markup and JS in the
dev server in external files rather than as string literals in
`serve-dev.js`. This enables them to be edited without having to restart
the dev server.

 - Extract shared functions for loading and unloading client into
   `scripts/util.js`

 - Move logic specific to the index template into `scripts/index.js`.

Adding a custom attribute without a `data-` prefix could potentially
cause headaches for publishers running their code through HTML
validation tools.

The toggle button only exists on the index page. Also I fixed an issue
where an incorrect variable name went unnoticed due to DOM element IDs
creating global variables (sigh).

Add a button to the dev server to facilitate easy testing of unloading
and re-loading the client, as done by the browser extension and also
some third-party pages.

Ensure that all links, script and stylesheet elements added to the page
by the boot script are removed when the client is unloaded.

This fixes a problem where the recently added `<link>` for the notebook
app was not removed when the page was unloaded, causing attempts to
activate the browser extension on a page where the client had previously
been loaded and then removed, to fail.

The implementation works by tagging all elements with a
`hypothesis-asset` attribute in the boot script and then removing these
elements when the client is unloaded.

On mobile devices, reseting the zoom level (pinch gesture) leaves a one pixel gap between the bucket-bar and the iframe. By adding one extra pixel to the width of the bucket-bar we eliminate this issue.

Add a pressure valve to keep from loading too many annotations in groups
that have a large number of annotations.

Replace iframe in `Notebook` if `groupId` changes

`load` was written with `SidebarView` in mind, assuming the presence
of both a `groupId` and one or more `uris`. This allows the loading of
all annotations that pertain to a specific group on one or more
specific document URIs.

For Notebook, we'd like to be able to, initially, load all annotations
for a `group` but for all document URIs.

Make `load` take `options` instead of individual search arguments. Leave
requirement for a `groupId` in place for now.

Bumps [postcss](https://github.com/postcss/postcss) from 8.1.9 to 8.1.10.
- [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/8.1.9...8.1.10)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Bumps [prettier](https://github.com/prettier/prettier) from 2.2.0 to 2.2.1.
- [Release notes](https://github.com/prettier/prettier/releases)
- [Changelog](https://github.com/prettier/prettier/blob/master/CHANGELOG.md)
- [Commits](https://github.com/prettier/prettier/compare/2.2.0...2.2.1)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>