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

Make `streamer` auto-reconnect on user change to support this migration

Avoid accessing state via `getState()`

This is needed so that `searchUris` don’t change when non-relevant
frame state changes

Add a selector to check if any annotation fetches have completed yet

Add selector to check if there are any applied filters on annotations

In preparation for the removal of angular-mocks from the codebase,
remove the final usage of it outside UI components.

Tune tutorial text to eliminate input-modality-specific instructions.