Extract thread-count functions into utility

Fix hidden text for highlights affecting text selection

The hidden "annotation start" and "annotation end" text added before and
after highlights was visually hidden using `opacity: 0` but still
interfered with text selection, presumably because the hidden text was
still "visible" to hit testing. Revise the approach to use the CSS
`clip` instead which avoids this problem.

Fixes https://github.com/hypothesis/support/issues/92

Bump fetch-mock from 9.0.0 to 9.1.1

Bump cross-env from 7.0.0 to 7.0.2

Bump terser from 4.6.4 to 4.6.6

Bump @sentry/browser from 5.13.0 to 5.13.2

Bumps [@babel/preset-env](https://github.com/babel/babel) from 7.8.6 to 7.8.7.
- [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/v7.8.6...v7.8.7)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Bumps [aws-sdk](https://github.com/aws/aws-sdk-js) from 2.630.0 to 2.635.0.
- [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/v2.630.0...v2.635.0)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Bumps [eslint-plugin-react](https://github.com/yannickcr/eslint-plugin-react) from 7.18.3 to 7.19.0.
- [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/v7.18.3...v7.19.0)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Bumps [@babel/core](https://github.com/babel/babel) from 7.8.6 to 7.8.7.
- [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/v7.8.6...v7.8.7)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Bumps [cross-env](https://github.com/kentcdodds/cross-env) from 7.0.0 to 7.0.2.
- [Release notes](https://github.com/kentcdodds/cross-env/releases)
- [Changelog](https://github.com/kentcdodds/cross-env/blob/master/CHANGELOG.md)
- [Commits](https://github.com/kentcdodds/cross-env/compare/v7.0.0...v7.0.2)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Bumps [@sentry/browser](https://github.com/getsentry/sentry-javascript) from 5.13.0 to 5.13.2.
- [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/5.13.0...5.13.2)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Bumps [terser](https://github.com/terser/terser) from 4.6.4 to 4.6.6.
- [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/v4.6.4...v4.6.6)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Bumps [fetch-mock](https://github.com/wheresrhys/fetch-mock) from 9.0.0 to 9.1.1.
- [Release notes](https://github.com/wheresrhys/fetch-mock/releases)
- [Commits](https://github.com/wheresrhys/fetch-mock/compare/v9.0.0...v9.1.1)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Freeze store state with `immutable` in development builds

Deep-freeze the store state after each action in development builds to
catch accidental mutation bugs such as #1879.

We couldn't do this in the past because the `annotation` component used
to mutate newly created annotations. As a result of recent refactoring,
this no longer happens.

Remove `unloadAnnotations` method from `annotation-mapper`

This method inadvertently (?) mutated annotations in the store which
caused a crash later. Ultimately this method dispatched an event which
was handled only in one place in `root-thread.js` to call
`store.removeAnnotations()`.

This commit removes the method and associated `ANNOTATIONS_UNLOADED`
event and replaces callers with direct calls to
`store.removeAnnotations(...)`.

All uses of Angular events will soon need to be removed as part of the
migration away from Angular JS, but that will happen separately.

Fixes #1879

Bump `eslint-config-hypothesis` and remove redundant `.eslintrc` rule

Upgrade `eslint-config-hypothesis` and remove the local `env` rule that
is handled by it.

Convert "adder" toolbar to Preact

The adder toolbar used to set the `display` style property on its
buttons to resolve an issue with a site that would add `display:none` to
them. The linked issue no longer occurs, possibly because the use of
Shadow DOM hides the buttons from `document.querySelectorAll(...)`.

A timeout that was used to add/remove the `is-active` class from the
toolbar appears to be unnecessary. The "grow and fade in" transition
when the toolbar is shown works without it.

To enable us to take advantage of the UI tooling that we built for the
sidebar app in the "annotator" part of the client (eg. components,
linting, testing infrastructure), we're going to convert UI components
in the "annotator" code over to Preact as well. As a first step, this
commit converts the "adder" toolbar that presents the "Annotate" and
"Highlight" buttons.

 - Extract the logic for creating a shadow DOM root for the adder into
   a separate module (`annotator/util/shadow-root.js`) so we can re-use it for other UI
   elements in future and remove Shadow DOM v0 support since Shadow DOM v1 has
   been available for a long time now.

 - Replace the `adder.html` template file, manual element manipulation
   and manual event listener setup with an `AdderToolbar` Preact
   component (`annotator/components/adder-toolbar.js`), which uses the same
   tooling as components in the sidebar app

In this commit the tests for all the pieces of the adder are covered by
`adder-test.js`. Future commits will split out tests for the individual
pieces.

Replace two dependencies that are only used once

Split `loadAnnotationsService` out of `annotationsService`

The `load` and `searchAndLoad` methods previously within
`annotationsService` didn't have a lot of functional or dependency
overlap with the other methods in `annotationsService`. Simplify
tests and make module responsibilities clearer by splitting
this into two, introducing a new `loadAnnotationsService`.

Update the `sidebar-content` controller to use
`loadAnnotationsService`.

Update browser extension after a new prod client release

Trigger a build of the "master" branch of the "browser-extension" project after
a new client production release with the `BUILD_TYPE` parameter set to
`update-hypothesis-client` to trigger an update to the Hypothesis client.

In implementing this, I ran into a couple of issues that caught me out
initially:

 - The `build` command must take a job name rather than a project (which
   is essentially a folder of jobs), ie. `browser-extension/master`
   rather than `browser-extension`. See [1].

 - To pass a value for a choice parameter, the easiest option is just to
   pass a string [2]

[1] https://stackoverflow.com/questions/46471467/
[2] https://stackoverflow.com/a/41760715/434243

Since any long-enough random string will do as an ID for the WebSocket
client, use the existing `generateHexString` function from
`util/random`.

In the one place we use it, this can be replaced with a trivial helper.

Remove debug code from production build

Refactor fetch-config.js and allow requestConfigFromFrame to be an object

- Rename fetchConfig to fetchConfigLegacy
- Allow the requestConfigFromFrame variable from hypothesisConfig to be either a string containing the origin url (legacy),
or an object containing the origin url + the ancestor level value of the parent frame. This is used to directly find the
frame which to send RPC requests too so we don't have to guess and create erroneous console errors.
- Rename fetchConfig to fetchConfigLegacy (now deprecated)
- Add fetchConfigRpc to replace fetchConfigLegacy
- Add fetchConfigEmbed for the simple hash fragment case
- Refactor fetchConfig to decide how it will fetch the host config (one of 3-ways)
 1. simple case from url hash fragment
 2. legacy RPC call from (depreciated)
 3. RPC call directly to parent frame.

Move `createAnnotation` to a service

Move `createAnnotation` out of the store thunk and into a service.
Add auto-creation of draft for new annotations.

Fixes #1834

AnnotationOmega: Add “Saving…” message

Re-implement “Saving…” message from legacy `Annotation` template

Fixes #1836