add `type-coercion.js` util with 4 methods. These are used on incoming the host-config values `openSidebar` and `requestConfigFromFrame` which may have an incompatible type upstream (via3) from what the client expects.

- toInteger
- toObject
- toString
- toBoolean

These methods coerces specific values into types so that downstream (client) methods can make assumptions. This is part of the prep work to support via3 which will eventually only export strings values in the config.

Revert mocha to v7.0.1

In mocha >= 7.1.0, exceptions thrown in test hooks (eg. a `beforeEach`
hook) cause the tests to hang without printing any useful exception
details.

From my notes in a Slack investion:

> The karma-mocha package is responsible for running mocha by calling
> mocha.setup() before the test modules are evaluated and then mocha.run()
> after they are evaluated to actually run the tests, as well as listening
> for test events from Mocha’s “runner” and reporting them back to Karma.
> When the beforeEach hook fails, the runner emits a “fail” event which
> karma-mocha receives. karma-mocha then attempts to emit a “test end”
> event and that ends up triggering an exception in a call to
> test.retriedTest() in code which was added in
> https://github.com/mochajs/mocha/pull/4181/files.

Until this issue is resolved, pin mocha to a version that doesn't
trigger this problem.

Replace `annotation-thread` with preact `Thread` component

Adjust CSS not contained within `Thread` to account for annotations
displayed within nested threads.

Exclude src/test-util from code coverage metrics

Our current policy is not to include test-only code in coverage metrics.

Add preStart method to cross-origin-rcp module

When called, allows any incoming rpc request to wait in a queue and be proceeded a short time later when the actual start() method is called. This prevents potential timeouts from incoming RPC requests from LMS (or similar) that arrive after we tipped off the parent frame with a fetchConfig outgoing request but before startServer() is called.

- Rename start() to startServer()
- Use newer module export standard that matches the rest of the app

Remove $location service and annotation subcomponents from sidebar entry module

Remove unused `permissions` service

After `Annotation` was converted to Preact, the subcomponents no longer
need to be wrapped and exposed to Angular.

This built-in Angular service is no longer used, so remove the code to
configure it and expose it to other services.

Use `permissions` util instead

Replace angular `toastr` with preact-compatible toast messages

Replace Angular router

Bump fetch-mock from 9.2.1 to 9.3.1

* preact-compatible
* more accessible

Fix an issue where the route was set before the initial profile and
groups fetch had completed, which could result in the WebSocket
connection being created and then immediately torn down and re-created
on startup.

The route was set too soon because Safari and Chrome emit a "popstate"
event on page load, which triggered a call to `router.sync()` before the
call to `sync()` made by `setupRoute` in src/sidebar/index.js after the
initial profile and groups load.

The solution is to only add the "popstate" listener after the initial
call to `sync()`.

Remove legacy `annotation` component; rename `AnnotationOmega` -> `Annotation`

See https://github.com/hypothesis/client/pull/1906#discussion_r394340280
and https://github.com/hypothesis/client/pull/1906#discussion_r394341565

Implement a replacement for the Angular JS router which will enable the
root components of the app to be more easily migrated to Preact in future.
Since the app only has three routes with very simple parameters, we
don't yet need a fully-fledged router implementation but can get by with
some pretty basic URL parsing.

 - Introduce a store module which holds the currently active route
   (`null`, "sidebar", "annotation" or "stream") and any parameters of
   that route.

   This state replaces the `isSidebar` boolean in the `viewer` module,
   since whether the app is the sidebar can be determined by checking
   the active route name.

 - Add a `router` service which handles synchronizing the store route
   with the current URL (via the `sync` method or in response to a
   `popstate` DOM event) or updating the URL and active route in the
   store (via the `navigate` method).

 - Modify the `<hypothesis-app>` component to fetch the active route
   from the store and use it to conditionally render the appropriate
   content component. This replaces the Angular router configuration
   that used to be in `src/sidebar/index.js`

 - Change the places that used to read or update the current route to
   read the active route and route params from the store (`store.{route,
   routeParams}`) and switch the current route by using the `router` service
   (`router.navigate`)

 - Remove the unused angular-route package

Fixes #1878

Fix spurious WebSocket disconnection errors

Bumps [fetch-mock](https://github.com/wheresrhys/fetch-mock) from 9.2.1 to 9.3.1.
- [Release notes](https://github.com/wheresrhys/fetch-mock/releases)
- [Commits](https://github.com/wheresrhys/fetch-mock/compare/v9.2.1...v9.3.1)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Remove browserify-shim

Revert "Bump prettier from 1.19.1 to 2.0.1"

This reverts commit 5cb9041d.