Only set `aria-label` and `title` attributes on `<button>` elements if
they are different from the text displayed in the button.

allowLeavingGroups is a new service level config setting that can globally override the the UI to hide the leave button for a group. This is necessary for section groups in LMS where groups are indeed private, but also can’t be left.

Cleanup sidebar CSS

This rule is already in the user-agent stylesheet for Safari, Chrome and
Firefox.

This doesn't belong here and we don't have a `<markdown>` element in the
app any more.

Refactor the styles used by the `HypothesisApp` component and other
top-level elements in the sidebar page to follow the conventions used by
other components. Also clean up some unused styles.

- Rename CSS classes for `HypothesisApp` component to `hypothesis-app*`
  to follow our normal naming conventions

- Consolidate styles for the `body` element in `sidebar.scss`

- Consolidate general element styles in `elements.scss`, eliminating
  `common.scss`

- Remove unnecessary styles for `html` and `body`

- Move `sheet` styles into `focused-mode-header` class, since these
  styles were only ever applied to that element

- Extract the styles from the `content` class used to center content in
  the top bar and content area into a mixin and then use that mixin in
  the component styles for the top bar and content area

- Remove unused mixin imports from `sidebar.scss`. These are no longer
  needed since the transition to the new SASS module system

Follow the module naming convention of other components.

Remove unused `USER_CHANGED` event

Improve document outline and stucture

Bump puppeteer from 3.0.1 to 3.0.2

Bump karma from 5.0.2 to 5.0.4

Bumps [karma](https://github.com/karma-runner/karma) from 5.0.2 to 5.0.4.
- [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/v5.0.2...v5.0.4)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Bump terser from 4.6.12 to 4.6.13

Bump karma-mocha from 2.0.0 to 2.0.1

Bumps [@babel/preset-env](https://github.com/babel/babel) from 7.9.5 to 7.9.6.
- [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/v7.9.5...v7.9.6)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Bumps [@babel/core](https://github.com/babel/babel) from 7.9.0 to 7.9.6.
- [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/v7.9.0...v7.9.6)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Bump postcss from 7.0.27 to 7.0.29

All consumers of this event have now been migrated to watch
`store.profile()` and/or `store.hasFetchedProfile()` instead.

Bumps [postcss](https://github.com/postcss/postcss) from 7.0.27 to 7.0.29.
- [Release notes](https://github.com/postcss/postcss/releases)
- [Changelog](https://github.com/postcss/postcss/blob/master/CHANGELOG.md)
- [Commits](https://github.com/postcss/postcss/compare/7.0.27...7.0.29)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Bumps [aws-sdk](https://github.com/aws/aws-sdk-js) from 2.663.0 to 2.669.0.
- [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/v2.663.0...v2.669.0)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Migrate HypothesisApp component and remove AngularJS

- Also add heading reset CSS so that headings can be used without fear
  of visual effects

- Add `@screen-reader-only` mixin
- Add `.u-screen-reader-only` utility class rules

Convert some `section` elements to `div`s for a less chattery document
structure.

Bumps [karma-mocha](https://github.com/karma-runner/karma-mocha) from 2.0.0 to 2.0.1.
- [Release notes](https://github.com/karma-runner/karma-mocha/releases)
- [Changelog](https://github.com/karma-runner/karma-mocha/blob/master/CHANGELOG.md)
- [Commits](https://github.com/karma-runner/karma-mocha/compare/v2.0.0...v2.0.1)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Bumps [terser](https://github.com/terser/terser) from 4.6.12 to 4.6.13.
- [Release notes](https://github.com/terser/terser/releases)
- [Changelog](https://github.com/terser/terser/blob/master/CHANGELOG.md)
- [Commits](https://github.com/terser/terser/compare/v4.6.12...v4.6.13)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Bumps [puppeteer](https://github.com/puppeteer/puppeteer) from 3.0.1 to 3.0.2.
- [Release notes](https://github.com/puppeteer/puppeteer/releases)
- [Commits](https://github.com/puppeteer/puppeteer/compare/v3.0.1...v3.0.2)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Also remove the `angular-util` helpers which are no longer used after
this change.

Convert the root `<hypothesis-app>` component to Preact and update the
startup code in `src/sidebar/index.js` to replace the AngularJS app
bootstrap with rendering the `HypothesisApp` component directly using
`render` from Preact.

This is no longer needed as we no longer have any AngularJS components
in the app.

Assorted Angular migration cleanups

Migrate `SidebarContent` to preact