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.

 - Use `is` prefix for boolean vars consistently

 - Note that the inline controls are optional

 - Clarify units of numeric props

 - Use parens to clarify precedence in a couple of expressions. This
   required overruling Prettier's formatting of these.

Convert the `Excerpt` component to Preact. Rather than convert the
existing implementation verbatim, this is a completely new implementation which
should be easier to understand and use. Instead of requiring callers to
provide an input property which represents the displayed data, which triggers a
re-measurement if it changes, the new implementation observes the DOM
directly for size changes.

This component renders caller-provided content (ie. it accepts a `children`
prop), which is not supported by the Preact <-> Angular bridge. Therefore it was
also necessary to create components (`AnnotationBody`, `AnnotationQuote`) that
encapsulate uses of `Excerpt` inside the `<annotation>` component.

 - Add `observe-element-size` utility module to watch for changes in
   the size of a DOM node using APIs available in the current browser

 - Add new `Excerpt` implementation and remove the old one

 - Remove `excerpt-overflow-monitor` utility that is not used by the new
   implementation

 - Add `AnnotationBody` component to render an annotation's markup body
   inside a (new) excerpt and convert the Angular template for
   `<annotation>` (annotation.html) to use it.

 - Add `AnnotationQuote` component to render an annotation's quote
   inside an excerpt and convert `annotation.html` to use it

Convert excerpt to Preact (1/n) - Add a utility to observe element size changes

Address some confusion I encountered while testing
https://github.com/hypothesis/lms/pull/1157/ where JSON-RPC messages
from the lms app to the client would be silently dropped if the
`rpcAllowedOrigins` config was not set correctly.

Log a console warning when this happens to make the problem more
obvious.  Before warning though, filter out non JSON-RPC messages
because there are various other sources of "message" events which the
client is likely to receive and which we don't want to result in warning
spam.

 - Set an `action` on the form so that the virtual keyboard shows a
   submit button, which makes it easier to trigger a search

 - Use a larger font size for readability and to prevent iOS from
   zooming into the field on desktop websites

 - Increase the font size of the input field to make the text easier to
   read, and more importantly, stop iOS from zooming into the text field
   on mobile-optimized sites (ie. those with `<meta name="viewport"
   content="width=device-width, initial-scale=1">` in the host page)

 - Make the toolbar buttons bigger when using touch input and add space
   below the toolbar so that the browser can differentiate taps on the
   buttons from taps on text selection handles

 - Prevent toolbar overflowing annotation card on small screens.
   We use either horizontal scrolling or wrapping depending on the input mode
   (touch/non-touch). Scrolling saves vertical space, but could be inconvenient
   to use with a mouse or touchpad.

fixup! - Update comments

Show focus ring on annotator toolbar buttons when using keyboard focus

[Security] Bump https-proxy-agent from 2.2.2 to 2.2.4

Reduce test startup time when running a subset of tests

Bumps [https-proxy-agent](https://github.com/TooTallNate/node-https-proxy-agent) from 2.2.2 to 2.2.4. **This update includes security fixes.**
- [Release notes](https://github.com/TooTallNate/node-https-proxy-agent/releases)
- [Commits](https://github.com/TooTallNate/node-https-proxy-agent/compare/2.2.2...2.2.4)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

This package provides polyfills for new ES/DOM APIs for use in IE 11.
This was held back to a v2.x version due to a conflict with fetch-mock
that is now resolved.

Bump fetch-mock from 6.5.2 to 8.0.0

The entry point for fetch-mock v8 is an ES module which Browserify does
not support natively. Therefore use the CommonJS entry point as
described at http://www.wheresrhys.co.uk/fetch-mock/#usageimporting.

Bump npm-packlist from 1.4.6 to 2.0.1

Bump @sentry/browser from 5.7.1 to 5.9.0

Bump sass from 1.23.3 to 1.23.6

Bump commander from 3.0.2 to 4.0.1

Bumps [sass](https://github.com/sass/dart-sass) from 1.23.3 to 1.23.6.
- [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/1.23.3...1.23.6)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Bumps [enzyme-adapter-preact-pure](https://github.com/preactjs/enzyme-adapter-preact-pure) from 2.1.0 to 2.2.0.
- [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/v2.1.0...v2.2.0)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Bumps [@sentry/browser](https://github.com/getsentry/sentry-javascript) from 5.7.1 to 5.9.0.
- [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/5.7.1...5.9.0)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Bumps [aws-sdk](https://github.com/aws/aws-sdk-js) from 2.556.0 to 2.573.0.
- [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/v2.556.0...v2.573.0)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Bumps [npm-packlist](https://github.com/npm/npm-packlist) from 1.4.6 to 2.0.1.
- [Release notes](https://github.com/npm/npm-packlist/releases)
- [Commits](https://github.com/npm/npm-packlist/compare/v1.4.6...v2.0.1)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Bumps [fetch-mock](https://github.com/wheresrhys/fetch-mock) from 6.5.2 to 8.0.0.
- [Release notes](https://github.com/wheresrhys/fetch-mock/releases)
- [Commits](https://github.com/wheresrhys/fetch-mock/compare/v6.5.2...v8.0.0)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>