Use a simpler mixin for Notebook's "inner" container: we don't need
all the stuff in `panel`, `card-frame` suffices. This eliminates the
need to override a vertical rhythm rule.

This is the first time in my life I've ever found what seems like a
potentially valid use case for `!important`: assuring that a utility
class that establishes rhythm based on margins is able to do its job.

Typically, our components should not be styling their own margins, but
there are some cases where we do, e.g. the initial reset of button
styles, which sets `margin: 0`. (Components should style their own
padding, but margins are the domain of containing/parent elements).

These two `!important` rules allow two utility mixins that are explicitly
concerned with margins to trump the specificity of margin-resetting
rules on descendent elements.

See https://css-tricks.com/when-using-important-is-the-right-choice/

This PR makes the adder to reposition when the adder is visible and the
window is resized. In this way the adder is always close to the selected
text.

Closes #3194

Use the `ListenerCollection` utility to register and unregister
listener, similarly to other parts of the application.

Add sidebar-wide custom styling for "compact" icon buttons: icon-only
buttons with no touch-input min-size and tighter padding, as used in
the `TopBar`

Update components to use `IconButton` with `CompactIconButton` styling
where applicable.

* Labeled buttons should continue sizing font as 1em, icons default at
  1.25em (no change)
* Icon-only buttons should size icon at 1em (instead of 1.25em) because
  the icon _is_ the content

Update `AnnotationActionBar` to set local font size to 16px to get
desired icon sizing (@1em).

This will allow elements to set contextual 1em=16px for appropriate
icon sizing (in buttons)

`GroupListItem` and `GroupListSection` are never used outside of the
`GroupList` itself.

Bumps [karma](https://github.com/karma-runner/karma) from 6.2.0 to 6.3.1.
- [Release notes](https://github.com/karma-runner/karma/releases)
- [Changelog](https://github.com/karma-runner/karma/blob/master/CHANGELOG.md)
- [Commits](https://github.com/karma-runner/karma/compare/v6.2.0...v6.3.1)
Signed-off-by: dependabot[bot] <support@github.com>

When the selection of text finishes on the sidebar's iframe, the click
event is not fired. Hence, the sidebar is not collapsed and the adder is
not displayed. This is because events don't bubble up through the
iframe.

This PR makes the sidebar to collapse on 'mousedown' events on the host
page. This is similar in nature to the 'touchstart' event that guest
already listens (although for a different reason).

Closes #463

Make sure all buttons—including icon-only buttons—get a flex layout. For
icon-only buttons, this ensures that the icon is centered on the button,
which is relevant especially when a `min-width` or `min-height` is
applied.

The term "plugin" is vestigial. The remaining cross-frame.js module in this
directory has been moved up to the top level of `src/annotator/` since
there isn't a more specific home that is suitable. Also change the
`CrossFrame` class to be a named rather than default export per current
conventions.

Move `RenderingStates` enum from `pdfjs-rendering-states.js` to
`src/annotator/anchoring/pdf`.

This is part of an exercise to declutter the root of src/annotator.

Bumps [@babel/preset-env](https://github.com/babel/babel/tree/HEAD/packages/babel-preset-env) from 7.13.10 to 7.13.12.
- [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/v7.13.12/packages/babel-preset-env)
Signed-off-by: dependabot[bot] <support@github.com>

Bumps [@babel/preset-react](https://github.com/babel/babel/tree/HEAD/packages/babel-preset-react) from 7.12.13 to 7.13.13.
- [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/v7.13.13/packages/babel-preset-react)
Signed-off-by: dependabot[bot] <support@github.com>

Bumps [@babel/core](https://github.com/babel/babel/tree/HEAD/packages/babel-core) from 7.13.10 to 7.13.13.
- [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/v7.13.13/packages/babel-core)
Signed-off-by: dependabot[bot] <support@github.com>

Bumps [mustache](https://github.com/janl/mustache.js) from 4.1.0 to 4.2.0.
- [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/v4.1.0...v4.2.0)
Signed-off-by: dependabot[bot] <support@github.com>

Bumps [npm-packlist](https://github.com/npm/npm-packlist) from 2.1.4 to 2.1.5.
- [Release notes](https://github.com/npm/npm-packlist/releases)
- [Commits](https://github.com/npm/npm-packlist/compare/v2.1.4...v2.1.5)
Signed-off-by: dependabot[bot] <support@github.com>

Bumps [@octokit/rest](https://github.com/octokit/rest.js) from 18.3.5 to 18.5.2.
- [Release notes](https://github.com/octokit/rest.js/releases)
- [Commits](https://github.com/octokit/rest.js/compare/v18.3.5...v18.5.2)
Signed-off-by: dependabot[bot] <support@github.com>

Bumps [eslint-plugin-react](https://github.com/yannickcr/eslint-plugin-react) from 7.22.0 to 7.23.1.
- [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/v7.22.0...v7.23.1)
Signed-off-by: dependabot[bot] <support@github.com>

Bumps [aws-sdk](https://github.com/aws/aws-sdk-js) from 2.868.0 to 2.874.0.
- [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/v2.868.0...v2.874.0)
Signed-off-by: dependabot[bot] <support@github.com>

Bumps [eslint](https://github.com/eslint/eslint) from 7.22.0 to 7.23.0.
- [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/v7.22.0...v7.23.0)
Signed-off-by: dependabot[bot] <support@github.com>

 - Capitalize acryonyms in variable names except for the first word
 - Update the description of a stub test

Explicitly define the common `Integration` interface which is
implemented by both `HTMLIntegration` and `PDFIntegration`.

This makes it more obvious what is part of the common interface that
`Guest` can depend on and what are additional properties or methods that
are specific to a particular implementation.

Make the HTML document integration structure consistent with the PDF document
integration by:

 - Extracting the integration implementation into a `HTMLIntegration`
   class in `src/annotator/integrations/html.js`
 - Renaming the document metadata extraction class from `DocumentMeta`
   to `HTMLMetadata` (to match PDFMetadata) and moving it to
   `src/annotator/integrations/html-metadata.js`

Per coding conventions recently agreed in the team, acronyms in symbol
names are capitalized unless they are the first word of a camelCased
symbol.

The name made it unclear what the boolean argument did.

Unify the code paths in the `Guest` class that invoke document
type/viewer-specific code such as getting the document URI and metadata
and anchoring and generating selectors.

 - Replace `pdfIntegration` and `documentMeta` fields in `Guest` with an
   `integration` field which provides the interface to document-type
   specific functionality
 - Replace the roundabout method of `PDFIntegration` customizing the
   anchoring logic with `anchor` and `describe` methods which the
   `Guest` calls as needed
 - Add documentation and tests for `uri` and `getMetadata` methods in
   `PDFIntegration`

Adjust the capitalization of the fake to match the name of the real
class.

Create an `integrations` directory within `src/annotator` which will
house the `*Integration` classes that encapsulate document
type/viewer-specific functionality, and create the first version of the
PDF integration by moving/renaming the existing `PDF` class. The
supporting `PDFMetadata` class has also been moved.

Per recently agreed code conventions [1], the `PDFIntegration` and
`PDFMetadata` classes are no longer default exports.

[1] https://github.com/hypothesis/frontend-toolkit/blob/master/docs/js-guide.md#default-exports

Add shared button components in a temporary "shared/components"
directory. The plan is to extract this to the `frontend-shared` package
once it has been more further integrated into the client preact app(s)
and the kinks are worked out. This will replace
`src/sidebar/components/Button` and will be reusable in other apps

Update project typecheck configuration so that typechecking may be
additionally applied to the `dev-server` area of the project. This helps
for typechecking usage of components in the "UI Playground" app.

* Add `tsconfig.json` for `dev-server` area of project
* Add "project references" at root level
  See https://www.typescriptlang.org/docs/handbook/project-references.html
* Update `package.json` `typecheck` script

Move the functionality that PdfSidebar implements elsewhere and remove
the class. This mainly consists of side-by-side mode, but there is also
the logic to get the scrollable document container for use with the
bucket bar. The PDF-specific logic has been moved to the `PDF` class in
`src/annotator/plugin/pdf.js` and the generic logic has been moved to
the `Sidebar` or `Guest` classes.

This change is a step towards creating an common interface between the
Guest/Sidebar and the document-type/viewer specific functionality. In
future the `PDF` class will evolve into the implementation of this
interface for PDFs. For HTML documents the logic is currently contained
directly in the `Guest` class and this will be extracted out into a
class that implements the same interface.

 - Move the PDF-specific parts of side-by-side mode into
   `PDF#fitSideBySide`
 - Move the generic parts of side-by-side mode into the `Sidebar` class.
   This will allow it to be reused for HTML documents or other document
   types in future
 - Move the logic for determining the scrollable container into
   `PDF#contentContainer`
 - Change the annotator setup code in `annotator/index.js` to always
   instantiate the `Sidebar` class, regardless of document type