Re-implement the sidebar's vertical toolbar

This is addressed by the `if @frame` logic a few lines down which only
adds the toolbar if we're providing our own frame.

Address PR feedback by rewriting several test descriptions and adding
comments.

This commit re-implements the vertical toolbar on the left edge of the sidebar
to make future changes to the toolbar UI easier (eg. some upcoming a11y changes)
and to decouple it from the rest of the annotator application (the toolbar and
`Sidebar`/`Guest` classes currently access each other's internals in a rather
haphazard way).

The new implementation has a similar code structure to the `Adder` toolbar that is
shown when selecting text. It consists of:

 1. A Preact component in `src/annotator/components/toolbar`
    which renders the toolbar UI according to the current state

 2. A controller class in `src/annotator/toolbar.js` which renders the
    Preact component into a container element and has provides properties
    that the `Sidebar` and `Guest` classes can use to update its state.

    The controller takes only the container element and callbacks
    associated with each button as inputs, so it no longer calls methods
    on the Guest/Sidebar.

    The sidebar and guest now update the toolbar UI by setting properties
    on the toolbar controller, rather than using a mixture of manual
    manipulation of the toolbar DOM and publishing events which get
    eventually translated to method calls on the toolbar.

    If we convert the remainder of the sidebar UI from jQuery/manual DOM
    updates to Preact in future, then we can remove the toolbar
    controller entirely and just render the toolbar component from
    within the sidebar component.

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