Use context API to pass Angular services to React components

Instead of handling passing of Angular services to components directly
in `wrapReactComponent`, instead make the Angular `$injector` service,
from which references to all other services can be obtained, available
to the component tree via a `ServiceContext` provider.

Child components can then get references to services either using the
`withServices` wrapper, which looks up services and passes them as
props, or by using `useContext(ServiceContext)` to get a reference to
the injector directly.

To support accessing Angular services from nested React components
without having to pass them through each parent, create a helper which
wraps a component to look up service dependencies via an injector
exposed using the context API, and inject the dependencies as props.

Introduce a new approach to mocking modules in tests

Remove fragment anchoring support

Bump @octokit/rest from 16.24.1 to 16.25.0

Bump mocha from 6.1.3 to 6.1.4

Bump sinon from 7.3.1 to 7.3.2

Bump karma from 4.0.1 to 4.1.0

Bump gulp from 4.0.0 to 4.0.1

Bump babel-plugin-istanbul from 5.1.1 to 5.1.2

Bump enzyme-adapter-preact-pure from 1.12.0 to 1.13.0

Bump preact from 10.0.0-alpha.4 to 10.0.0-beta.0

Bumps [karma](https://github.com/karma-runner/karma) from 4.0.1 to 4.1.0.
- [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/v4.0.1...v4.1.0)
Signed-off-by: dependabot[bot] <support@dependabot.com>

Bumps [enzyme-adapter-preact-pure](https://github.com/preactjs/enzyme-adapter-preact-pure) from 1.12.0 to 1.13.0.
- [Release notes](https://github.com/preactjs/enzyme-adapter-preact-pure/releases)
- [Changelog](https://github.com/preactjs/enzyme-adapter-preact-pure/blob/master/CHANGELOG.md)
- [Commits](https://github.com/preactjs/enzyme-adapter-preact-pure/compare/v1.12.0...v1.13.0)
Signed-off-by: dependabot[bot] <support@dependabot.com>

Bumps [@octokit/rest](https://github.com/octokit/rest.js) from 16.24.1 to 16.25.0.
- [Release notes](https://github.com/octokit/rest.js/releases)
- [Commits](https://github.com/octokit/rest.js/compare/v16.24.1...v16.25.0)
Signed-off-by: dependabot[bot] <support@dependabot.com>

Bumps [sinon](https://github.com/sinonjs/sinon) from 7.3.1 to 7.3.2.
- [Release notes](https://github.com/sinonjs/sinon/releases)
- [Changelog](https://github.com/sinonjs/sinon/blob/master/CHANGELOG.md)
- [Commits](https://github.com/sinonjs/sinon/compare/v7.3.1...v7.3.2)
Signed-off-by: dependabot[bot] <support@dependabot.com>

Bumps [babel-plugin-istanbul](https://github.com/istanbuljs/babel-plugin-istanbul) from 5.1.1 to 5.1.2.
- [Release notes](https://github.com/istanbuljs/babel-plugin-istanbul/releases)
- [Changelog](https://github.com/istanbuljs/babel-plugin-istanbul/blob/master/CHANGELOG.md)
- [Commits](https://github.com/istanbuljs/babel-plugin-istanbul/compare/v5.1.1...v5.1.2)
Signed-off-by: dependabot[bot] <support@dependabot.com>

Bumps [gulp](https://github.com/gulpjs/gulp) from 4.0.0 to 4.0.1.
- [Release notes](https://github.com/gulpjs/gulp/releases)
- [Changelog](https://github.com/gulpjs/gulp/blob/master/CHANGELOG.md)
- [Commits](https://github.com/gulpjs/gulp/compare/v4.0.0...v4.0.1)
Signed-off-by: dependabot[bot] <support@dependabot.com>

Bumps [preact](https://github.com/developit/preact) from 10.0.0-alpha.4 to 10.0.0-beta.0.
- [Release notes](https://github.com/developit/preact/releases)
- [Commits](https://github.com/developit/preact/compare/10.0.0-alpha.4...10.0.0-beta.0)
Signed-off-by: dependabot[bot] <support@dependabot.com>

Bumps [mocha](https://github.com/mochajs/mocha) from 6.1.3 to 6.1.4.
- [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/v6.1.3...v6.1.4)
Signed-off-by: dependabot[bot] <support@dependabot.com>

Handle illegal group link states

Previously the visibleThreadList state in thread-list did not get
calculated in onInit because the method that did so was subscribed to
after the 'changed' event was emitted. This issue was resolved by
calling the method to calculate the state explicitly in onInit and
only subscribing the event handler for changes that may occur after init.

Add sidebar-content-error react component

dom-anchor-fragment is shipped as a UMD module which causes problems on
pages that declare a global function called `define` (see #479).

While we could fix the module, fragment anchoring is not very useful at
the moment. The client primarily anchors to the quote and uses other
anchors only as an optimization. The Wikipedia baseline tests changed in
this PR show how little value a fragment has on real websites.
Therefore, just remove support for generating and using fragment selectors.

This change should not cause anchoring regressions, as anchoring would
only have succeeded previously if the quote selector matched. Now it will just
fall through to anchoring using the range, position or quote selectors instead.

Fixes #479

Make group selectors more efficient

Ensure that selectors for getting the "My groups", "Currently viewing
groups" and "Featured groups" lists only recompute when the groups state
changes. Previously they would have recomputed when _any_ part of the state changed.

More generally, we should be using `createSelector` rather than
`memoize` whenever we have a place in the app that needs to extract a subset
of data from some larger structure and derive some data from that.

Bump @octokit/rest from 16.23.2 to 16.24.1

Bump mocha from 6.1.2 to 6.1.3

Bump prettier from 1.16.4 to 1.17.0

Bump jquery from 3.3.1 to 3.4.0

Bump enzyme-adapter-preact-pure from 1.10.4 to 1.12.0