Enable throttling of concurrent builds

Jenkins CI builds of the client project are prone to failing when many
concurrent builds occur. This often happens when many Dependabot PRs are
submitted at once during a weekly dependency update. These builds
succeed most of the time when retried.

Limiting the number of concurrent builds per executor can be done globally,
but it can also be done on a per-project basis using the
throttle-concurrents [1] plugin. Assign a throttling category to client
builds so that this latter approach can be used.

[1] https://plugins.jenkins.io/throttle-concurrents

Improve accessability for toggle buttons

- Add aria attribute for show / hide side bar
- Add aria attribute for show / hide highlights
- Fix a bug that prevents the correct class from showing on the highlight icon when highlights are initially disabled

Share Annotations Panel 3 & 4 of 4: Add `ShareAnnotationsPanel` and replace `ShareDialog` with it

- Remove old angular component and supporting modules/files
- Update `hypothesis-app` component
- Update `TopBar` component

- Perform minor refactor on CSS and remove unnecessary rule

Disable verbose Karma logging

This adds a lot of noise to the Jenkins log and we've gotten all the value we
can out of this for tracking down Jenkins CI build failures. What remains is a
period of time between Karma creating the `context.html` iframe where the test
code executes and the scripts in that iframe starting to actually execute.

During this time neither the Karma logs nor Chrome's verbose log provide any
insight into what is going on. Chrome tracing however does, so that's where the
investigation is continuing.

Bump npm-packlist from 1.4.4 to 1.4.6

Share Annotations Panel 2 of 4: Add base `SidebarPanel` component and store module

Bumps [@octokit/rest](https://github.com/octokit/rest.js) from 16.32.0 to 16.33.0.
- [Release notes](https://github.com/octokit/rest.js/releases)
- [Commits](https://github.com/octokit/rest.js/compare/v16.32.0...v16.33.0)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Bumps [dompurify](https://github.com/cure53/DOMPurify) from 2.0.4 to 2.0.5.
- [Release notes](https://github.com/cure53/DOMPurify/releases)
- [Commits](https://github.com/cure53/DOMPurify/compare/2.0.4...2.0.5)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Bumps [aws-sdk](https://github.com/aws/aws-sdk-js) from 2.543.0 to 2.545.0.
- [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/v2.543.0...v2.545.0)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Bumps [@babel/core](https://github.com/babel/babel) from 7.6.2 to 7.6.3.
- [Release notes](https://github.com/babel/babel/releases)
- [Changelog](https://github.com/babel/babel/blob/master/CHANGELOG.md)
- [Commits](https://github.com/babel/babel/compare/v7.6.2...v7.6.3)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Bumps [npm-packlist](https://github.com/npm/npm-packlist) from 1.4.4 to 1.4.6.
- [Release notes](https://github.com/npm/npm-packlist/releases)
- [Commits](https://github.com/npm/npm-packlist/compare/v1.4.4...v1.4.6)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Bumps [@babel/preset-react](https://github.com/babel/babel) from 7.0.0 to 7.6.3.
- [Release notes](https://github.com/babel/babel/releases)
- [Changelog](https://github.com/babel/babel/blob/master/CHANGELOG.md)
- [Commits](https://github.com/babel/babel/compare/v7.0.0...v7.6.3)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Bumps [@babel/preset-env](https://github.com/babel/babel) from 7.6.2 to 7.6.3.
- [Release notes](https://github.com/babel/babel/releases)
- [Changelog](https://github.com/babel/babel/blob/master/CHANGELOG.md)
- [Commits](https://github.com/babel/babel/compare/v7.6.2...v7.6.3)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Bumps [enzyme-adapter-preact-pure](https://github.com/preactjs/enzyme-adapter-preact-pure) from 2.0.2 to 2.1.0.
- [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/v2.0.2...v2.1.0)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Bumps [eslint-plugin-react](https://github.com/yannickcr/eslint-plugin-react) from 7.14.3 to 7.16.0.
- [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/v7.14.3...v7.16.0)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Enable Chrome tracing during startup

This enables very detailed logging of network activity, page execution
and script execution for 7 seconds after startup.

This is part of debugging frequent CI build failures in the client repo
on Jenkins. From the Karma logs I can see that all of the test files are
requested from the Karma server, but none of the log messages associated
with test execution are seen. This tracing should make it possible to
find out which JS scripts even execute.

Share Annotations Panel 1 of 4: Update icons; allow inline icons

Change SASS processing pipeline to match h, lms projects

Don't built the client a second time when running "npm publish"

Bump gulp-changed from 4.0.1 to 4.0.2

Bump autoprefixer from 9.6.1 to 9.6.4

Bump aws-sdk from 2.538.0 to 2.543.0

Bump cross-env from 6.0.0 to 6.0.3