Prepare SASS files for transition to SASS modules

Update icons in `MarkdownEditor`; add Help Link

Merge in recent changes to app styles.

Extract `ShareLinks` component

Fix conflicts in excerpt.scss

Migrate .scss files to SASS modules using:

```
sass-migrator module --load-path=node_modules/ --migrate-deps
src/styles/annotator/annotator.scss src/styles/sidebar/sidebar.scss
```

This was previously done for the lms app in https://github.com/hypothesis/lms/pull/1057

This defines `u-*` classes which are used in various places.

Nested imports only support loading plain CSS when using SASS modules.
Therefore move the bucket bar imports to the top level, but wrap the
styles inside that module in `.annotator-frame` to ensure they take
precedence over the CSS reset, which is the original reason for using a
nested import.

This module was used by multiple SASS entry points (annotator, sidebar)
even though each mixin or class within it was either trivial or only
actually used by one of the entry points. Moving the mixins to the
individual entry points will make it easier to run the migrator.

Fix margins on annotation body content causing content to be cropped

In preparation for running the sass-migrator tool, hoist several imports
from included SASS files up to the entry points at
src/styles/{annotator/annotator.scss, sidebar/sidebar.scss}.

This path was always wrong but happened not to break the build
previously.

Fix an issue where elements in the annotation body that were styled with
margins, such as headings and block quotes, could cause the bottom of an
excerpt to cropped.

The issue was that margin collapsing [2] caused the margin to "leak" out of
the excerpt's content `<div>` and push it vertically away from the outermost
element of the excerpt. The `max-height` applied to the outermost
element took into account the content's _height_ but not its vertical
offset.

The fix here is to make the excerpt content div a new block-formatting
context [1] which ensures that any margins on elements inside the
content are placed _inside_ the content element. There are various ways
of doing this, but `display: inline-block` is backwards compatible down
to IE 11.

 - Fix margin collapsing issue by making content a new block formatting
   context
 - Remove unused `excerpt__container` class and an incorrect comment
   about the excerpt class
 - Remove an unnecessary `test-name` attribute on the content element and
   use the class name to find it in tests instead
 - Add a note about some refactoring that can be done post SASS-module
   adoption

[1] https://developer.mozilla.org/en-US/docs/Web/Guide/CSS/Block_formatting_context
[2] https://developer.mozilla.org/en-US/docs/Web/CSS/CSS_Box_Model/Mastering_margin_collapsing

Fixes #1518

Factor out some reusable CSS into mixins

Extract some reusable button styles, as well as some patterns
for lists of linked icons and a "panel" pattern. It is likely
organization of this will evolve as a pattern-library approach
is implemented, but this avoids copy-pasta and tightens up
consistency between uses.

Bumps [angular](https://github.com/angular/angular.js) from 1.7.8 to 1.7.9. **This update includes a security fix.**
- [Release notes](https://github.com/angular/angular.js/releases)
- [Changelog](https://github.com/angular/angular.js/blob/master/CHANGELOG.md)
- [Commits](https://github.com/angular/angular.js/compare/v1.7.8...v1.7.9)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Revert "Update core-js to current version"

Remove unused form and button classes

Remove deprecated SCSS `$brand-color` color variable

Use `$brand` instead, which has the identical value

Update core-js to current version

Improve mobile UX of markdown editor and search input

Convert excerpt to Preact (2/n)

The prop name `overflowHysteresis` was frequently typo-d and is also
misleading as the excerpt doesn't remember the history of the overflow
value, it just uses it as a fixed threshold. Rename it to
`overflowThreshold` instead.

Also improve / simplify a few JSDoc comments per PR feedback.

Warn in JSON-RPC server if message is dropped by origin filter

Remove (some) deprecated SCSS color variables

`$color-cardinal` and `$gray-lightest` have no references in existing
SCSS

Also remove now-unused utility class for cancel buttons—this pattern
is only used in one place and may be on the way out in general.

Sub with `$grey-4`, which is identical (`#a6a6a6`). Part of consolidating
CSS colors.

Sub with `$grey-1`, which is virtually identical (`$color-seashell` was
`#f1f1f1`, `$grey-1` is `#f2f2f2`. Part of consolidating CSS colors.