This addresses PR feedback.

The visible part of PDF highlights are created by SVG elements which are
multiply-blended with the PDF content to enhance legibility of
highlighted text.  The blueish background for focused highlights was
still being applied to the transparent text layer on top of the PDF
which contains the `<hypothesis-highlight>` elements. This impacted the
legibility of focused highlights.

Improve the readability of PDF highlights by instead modifying the fill
color of the SVG element.

Bumps [@babel/preset-env](https://github.com/babel/babel/tree/HEAD/packages/babel-preset-env) from 7.13.12 to 7.13.15.
- [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/v7.13.15/packages/babel-preset-env)
Signed-off-by: dependabot[bot] <support@github.com>

Bumps [core-js](https://github.com/zloirock/core-js/tree/HEAD/packages/core-js) from 3.10.0 to 3.10.1.
- [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/commits/v3.10.1/packages/core-js)
Signed-off-by: dependabot[bot] <support@github.com>

Bumps [postcss](https://github.com/postcss/postcss) from 8.2.9 to 8.2.10.
- [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/8.2.9...8.2.10)
Signed-off-by: dependabot[bot] <support@github.com>

Bumps [@babel/core](https://github.com/babel/babel/tree/HEAD/packages/babel-core) from 7.13.14 to 7.13.15.
- [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/v7.13.15/packages/babel-core)
Signed-off-by: dependabot[bot] <support@github.com>

Bumps [eslint-plugin-react](https://github.com/yannickcr/eslint-plugin-react) from 7.23.1 to 7.23.2.
- [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/v7.23.1...v7.23.2)
Signed-off-by: dependabot[bot] <support@github.com>

Bumps [aws-sdk](https://github.com/aws/aws-sdk-js) from 2.880.0 to 2.884.0.
- [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/v2.880.0...v2.884.0)
Signed-off-by: dependabot[bot] <support@github.com>

Bumps [typescript](https://github.com/Microsoft/TypeScript) from 4.2.3 to 4.2.4.
- [Release notes](https://github.com/Microsoft/TypeScript/releases)
- [Commits](https://github.com/Microsoft/TypeScript/compare/v4.2.3...v4.2.4)
Signed-off-by: dependabot[bot] <support@github.com>

Bumps [enzyme-adapter-preact-pure](https://github.com/preactjs/enzyme-adapter-preact-pure) from 3.0.0 to 3.1.0.
- [Release notes](https://github.com/preactjs/enzyme-adapter-preact-pure/releases)
- [Changelog](https://github.com/preactjs/enzyme-adapter-preact-pure/blob/master/CHANGELOG.md)
- [Commits](https://github.com/preactjs/enzyme-adapter-preact-pure/compare/v3.0.0...v3.1.0)
Signed-off-by: dependabot[bot] <support@github.com>

Bumps [katex](https://github.com/KaTeX/KaTeX) from 0.13.0 to 0.13.2.
- [Release notes](https://github.com/KaTeX/KaTeX/releases)
- [Changelog](https://github.com/KaTeX/KaTeX/blob/master/CHANGELOG.md)
- [Commits](https://github.com/KaTeX/KaTeX/compare/v0.13.0...v0.13.2)
Signed-off-by: dependabot[bot] <support@github.com>

Bumps [eslint](https://github.com/eslint/eslint) from 7.23.0 to 7.24.0.
- [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/v7.23.0...v7.24.0)
Signed-off-by: dependabot[bot] <support@github.com>

In issue discussions, tests and various parts of the code there are
mentions of side-by-side mode being active or not. The `Guest` class
didn't store this state directly but instead had a
`closeSidebarOnDocumentClick` state which is always the inverse of
"is side-by-side active?".

Make the code easier to follow by storing the "is side-by-side active?" state
directly. This will also avoid a misnamed variable if in future the
state has other effects besides changing whether clicking on the
document closes the sidebar.

 - Replace `closeSidebarOnDocumentClick` with a private
   `_sideBySideActive` property, exposed via a read-only
   `sideBySideActive` getter

 - Refactor tests to only use the public API of Guest (calling
   `Guest#fitSideBySide` or reading `sideBySideActive`)

Remove the fallback logic that uses `location.href` and `document.title`
as the document URL and metadata if using the integration-specific code
(HTML or PDF) to get the document URL and metadata fails. Instead just
let the async exception propagate.  Currently this will result in no
annotation/highlight being created and a console error. In future we
should improve this by showing an error notification to the user.

Recording the wrong document metadata with an annotation is worse than
not creating an annotation at all: The user will be unaware that there
is a problem yet the annotation will likely fail to show up in future or
be displayed with completely wrong metadata in the Notebook. Therefore
we want the failure here to be obvious so that the user notifies us of
the problem and doesn't create a bunch of annotations which later
"disappear".

Fixes #3204

Add test to check that anchors not associated with the removed
annotation are preserved.

Rewrite the `Guest#anchor` method to simplify the control flow and
generally make it easier to understand and change. There are no
functional changes for annotations with zero or one entry in the
`target` field (all existing Hypothesis annotations).

There is a functional change to handling of annotations with multiple
targets. Previously `anchor` would try to be smart about only
re-anchoring targets which were not already anchored. In the new
implementation all targets for an annotation are re-anchored. This will have
no effect in practice because the Hypothesis client only supports
creating annotations with a single target and the h backend can only
store a single target per annotation. However the Hypothesis API does
allow for multiple targets per annotation, in line with the W3C Web
Annotations specs in which a single annotation can refer to multiple
parts of a document.

 - Convert `anchor` method to async and replace Promise chains with async/await

 - Replace the logic that removes existing anchors and highlights for an
   annotation with a call to the `detach` method. This required adding
   an internal parameter to `detach` to control whether `anchorsChanged`
   is emitted, so that `anchor` only emits `anchorsChanged` once.

 - Add an explicit error to `Promise.reject` calls in tests so that the
   tests are easier to debug if they fail

Devise a way to "extend" a common base playground app with local-app
routes and patterns

In the soon-future, we'll want to extract the common bits of the
UI Playground app to the `frontend-shared` repository. This is step 1.

Only `@forward` specific mixins and create a function to retrieve
color settings out of the `_config` partial

If opening the login popup window using `window.open` fails or succeeds
but fails to return a `Window` reference to the caller then show a more
helpful error message.

There are various rare scenarios (browser bugs, browser extensions)
which might cause this to happen. There is not a whole lot we can do to
recover, but at least we can show a better error message.

Use `IconButton` in `ShareAnnotationsPanel` and
`AnnotationShareControl`, with custom styling to make
the buttons pair well with a text input.

It would be ideal to create a design pattern from the
combo of the input + button in the future.

Replace uses of `Button` in the Thread component and adjust
styling for stronger alignment in both desktop and touch interfaces.

Add a full novel as a test document for testing performance and
behaviors related to long PDF documents, such as re-anchoring when pages
are rendered and un-rendered while scrolling through the document.

We try to keep example PDFs small to avoid bloating the git repository, but
this one is on the larger side (~700KB) due to its length.

This PDF was produced by downloading the EPUB version of the novel [1]
from Project Gutenberg and then converting it to a PDF using Calibre [2]

[1] https://www.gutenberg.org/ebooks/64317
[2] https://calibre-ebook.com

The types for `SortBy` and `SortOrder` were swapped.

This matches how Prettier formats self-closing JSX tags and makes
expressions such as `<Widget expanded />` more readable.

 - Indent children by two spaces

 - Handle boolean props closer to how we normally set them in JSX by omitting
   them if false and including them without explicit values if true.
   This assumes that boolean props default to false. If there is a
   scenario in future where this is not the case, this behavior may need
   to be configurable.

 - Handle `bigint` numeric values. We have no use case for this, but
   TypeScript pointed out that it wasn't handled.

 - Add JSDoc comments