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

Remove browserify-shim

Revert "Bump prettier from 1.19.1 to 2.0.1"

This reverts commit 5cb9041d.

Add selectors for direct-linked annotation, group IDs

Bumps [prettier](https://github.com/prettier/prettier) from 1.19.1 to 2.0.1.
- [Release notes](https://github.com/prettier/prettier/releases)
- [Changelog](https://github.com/prettier/prettier/blob/master/CHANGELOG.md)
- [Commits](https://github.com/prettier/prettier/compare/1.19.1...2.0.1)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Bump @sentry/browser from 5.14.2 to 5.15.0

This is obsolete with the current version of the "hammerjs" package.

As a result of evolution of the libraries involved, this build tool is no
longer required.

A minor change was required in one test to adjust to the fact that
Angular now uses jqLite, as it does in the actual application, instead of jQuery.
This is because "jquery" is no longer automatically added to the bundle
when "angular" is added.

Remove "raf" dependency

Bump mocha from 7.1.0 to 7.1.1

Bumps [mocha](https://github.com/mochajs/mocha) from 7.1.0 to 7.1.1.
- [Release notes](https://github.com/mochajs/mocha/releases)
- [Changelog](https://github.com/mochajs/mocha/blob/master/CHANGELOG.md)
- [Commits](https://github.com/mochajs/mocha/compare/v7.1.0...v7.1.1)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Bumps [@babel/preset-env](https://github.com/babel/babel) from 7.8.7 to 7.9.0.
- [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/v7.8.7...v7.9.0)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Bumps [eslint-plugin-react-hooks](https://github.com/facebook/react/tree/HEAD/packages/eslint-plugin-react-hooks) from 2.5.0 to 2.5.1.
- [Release notes](https://github.com/facebook/react/releases)
- [Changelog](https://github.com/facebook/react/blob/master/CHANGELOG.md)
- [Commits](https://github.com/facebook/react/commits/HEAD/packages/eslint-plugin-react-hooks)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Bumps [aws-sdk](https://github.com/aws/aws-sdk-js) from 2.641.0 to 2.643.0.
- [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/v2.641.0...v2.643.0)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Bumps [@babel/preset-react](https://github.com/babel/babel) from 7.8.3 to 7.9.1.
- [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/v7.8.3...v7.9.1)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Bumps [@babel/core](https://github.com/babel/babel) from 7.8.7 to 7.9.0.
- [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/v7.8.7...v7.9.0)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Bumps [@octokit/rest](https://github.com/octokit/rest.js) from 17.1.0 to 17.1.1.
- [Release notes](https://github.com/octokit/rest.js/releases)
- [Commits](https://github.com/octokit/rest.js/compare/v17.1.0...v17.1.1)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Bumps [@sentry/browser](https://github.com/getsentry/sentry-javascript) from 5.14.2 to 5.15.0.
- [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/5.14.2...5.15.0)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Remove feature-flag switch: Render `AnnotationOmega` in all cases

Fix browser warnings about an abnormal disconnection when the WS
connection is closed normally by calling `socket.close()`.

When `WebSocket#close` is called with no code, the browser will send a
close frame to the server with no payload. This is valid. On the backend
ws4py translates this into a 1005 status code, which is also correct.
However, ws4py then responds with the server's close frame and instead
of sending no payload, it sends a close frame with a payload that has a
1005 status. This is not allowed by the spec and results in errors in
the browser about an abnormal disconnection.

Although the proper fix for this needs to happen on the server, the
simplest solution right now is to set a status code in the client and
ws4py will respond with the same status code in the server's close
frame.

As part of this, refactor the tests to clearly separate the tests for
what happens when the browser/server closes the connection vs when the
client closes the connection.

Don't warn if the WebSocket is closed with a "going away" code (1001).
This is a "normal" closure and, per the WebSocket RFC [1], can happen
when the browser navigates away from the current page.

[1] https://tools.ietf.org/html/rfc6455#section-7.4.1

This used to have an effect when we used proxyquire for mocking. It does
nothing now.

requestAnimationFrame is supported natively [1] by all the browsers we
support.

[1] https://caniuse.com/#search=requestanimationframe

Add keyboard navigation to MarkdownEditor to improve a11y

- `arrow left`, `arrow right`, `home` and `end` keys are all supported and modeled after https://www.w3.org/TR/wai-aria-practices/examples/toolbar/toolbar.html

- Add roving tabIndex value for all toolbar elements.

This is a pre-requisite to removing the feature flag

This is part of eliminating direct access to store state, and a tiny
piece of simplifying the `groups` service.

Remove "invisible threads" logic

Remove "extend" dependency

This had been replaced with object spread or `Object.assign` apart from
in one instance.

Tune virtual thread list debouncing

When scrolling through a long list of annotations quickly, the sidebar
could display blank space for a long period of time. This was happening
because the debounced update function waited for a period of at least
20ms with no "scroll" events, and a gap of that length could take some
time to appear if there was a queue of scroll events being delivered.

Fix the issue by adding an upper bound of 100ms after a scroll event before
recalculating the list of visible threads.

Also make sure to cancel any pending updates after the `detach` method
is called.

This used to be required in order to run logic in the `<annotation>`
component that saved new highlights to the server, in the case where the
highlight's annotation card was off-screen. This is no longer required
since that is now handled by the `autosave` service.

Allow tab to submit tag

Run `yarn upgrade` to fix GitHub security alerts

Fix `acorn` security vulnerability warning by upgrading all packages to
current compatible versions using `yarn upgrade`.

After this, I also pruned some unused dependencies by running:

```
rm -rf node_modules
yarn
```