This adds documentation for how to build the docs locally.

Previously the `make docs` command required you to have Python 3 and tox
installed system-wide, neither of which are required for any of our
other projects anymore, and this wasn't documented anywhere.

This PR changes `make docs` to use pyenv to install Python 3 and tox, as
all our other Python projects do, and adds a Building the Docs Locally
page to the docs with instructions on how to install pyenv and build the
docs locally.

pyenv and tox are _not_ added to the general client dev install
instructions since they're only needed to build the docs.

`npm` is still used by the final deployment process to publish packages
to the npm registry.

Switch default browser for running tests from PhantomJS to headless Chrome

These are no longer needed when running tests under Chrome.

Update our test/CI environment to a modern browser which matches what
our users use and supports modern web APIs and ES language features.

 - Add a Dockerfile which defines an image that includes everything
   needed to run client tests and deployment steps. This image uses
   plain Alpine as the base rather than Debian as this makes it easier
   to update frequently (every 6 months) to get a current Chrome build.

 - Replace PhantomJS with Puppeteer for testing locally and on Travis.
   Jenkins uses Chromium packages from Alpine.

- Fix failing `services/api.js` test

  Chrome throws an error when trying to construct a fetch `Response` with
  a non-null `body` if the status is 204.

- Fix Timestamp and GroupListItem test failures in Chrome

  - Pass required type argument to `Event` constructor
  - Pass valid `timestamp` argument to `Timestamp`. This value is passed
    on to date functions which require a valid date/time string.

- Fix Slider transition test in Chrome

  Since the transition from open to closed is animated, the actual element
  height doesn't immediately transition to 0px. Instead of checking the
  actual height, check the target height specified by the current style
  properties.

  The previous code happened to work under PhantomJS, but was conceptually incorrect.

Change focus mode button label

Change the label when showing everyone's annotations from "Only <name>"
to "Show only <name>". Having the label always start with a verb is more
consistent and hopefully a bit clearer.

Remove "X" icon from toggle button in focus mode

Since clicking the button toggles between two states rather than
dismissing the "focus mode" header, the "X" icon doesn't feel
appropriate, so remove it.

The button's padding has been adjusted to center align the content
following the removal of the icon.

Fixes #1294

Add first iteration of UI to user-focused mode

Bump sinon from 7.3.2 to 7.4.1

Bump browserify from 16.3.0 to 16.5.0

Bump commander from 2.20.0 to 3.0.0

Bump enzyme-adapter-preact-pure from 2.0.1 to 2.0.2

Bump eslint-plugin-react-hooks from 1.6.1 to 1.7.0

Bumps [eslint-plugin-react-hooks](https://github.com/facebook/react/tree/HEAD/packages/eslint-plugin-react-hooks) from 1.6.1 to 1.7.0.
- [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 [enzyme-adapter-preact-pure](https://github.com/preactjs/enzyme-adapter-preact-pure) from 2.0.1 to 2.0.2.
- [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/v2.0.1...v2.0.2)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Bumps [browserify](https://github.com/browserify/browserify) from 16.3.0 to 16.5.0.
- [Release notes](https://github.com/browserify/browserify/releases)
- [Changelog](https://github.com/browserify/browserify/blob/master/changelog.markdown)
- [Commits](https://github.com/browserify/browserify/compare/v16.3.0...v16.5.0)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Bumps [commander](https://github.com/tj/commander.js) from 2.20.0 to 3.0.0.
- [Release notes](https://github.com/tj/commander.js/releases)
- [Changelog](https://github.com/tj/commander.js/blob/master/CHANGELOG.md)
- [Commits](https://github.com/tj/commander.js/compare/v2.20.0...v3.0.0)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Bumps [sinon](https://github.com/sinonjs/sinon) from 7.3.2 to 7.4.1.
- [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/v7.3.2...v7.4.1)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Update user-focus store, rootThread intergration for clarity

Add focused user mode for speed grader

Minor change to the component to add a button that enables or disables the focused mode for the focusedUser passed into the settings object.

Merge pull request #1287 from hypothesis/dependabot/npm_and_yarn/babel-plugin-transform-async-to-promises-0.8.14

Bump babel-plugin-transform-async-to-promises from 0.8.13 to 0.8.14

Bump preact from 10.0.0-rc.0 to 10.0.0-rc.1

Bumps [preact](https://github.com/developit/preact) from 10.0.0-rc.0 to 10.0.0-rc.1.
- [Release notes](https://github.com/developit/preact/releases)
- [Commits](https://github.com/developit/preact/compare/10.0.0-rc.0...10.0.0-rc.1)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Bumps [babel-plugin-transform-async-to-promises](https://github.com/rpetrich/babel-plugin-transform-async-to-promises) from 0.8.13 to 0.8.14.
- [Release notes](https://github.com/rpetrich/babel-plugin-transform-async-to-promises/releases)
- [Commits](https://github.com/rpetrich/babel-plugin-transform-async-to-promises/compare/v0.8.13...v0.8.14)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Add edited timestamp to annotation headers

Update the `time` utility module to more closely match
our current conventions. Also make time math consistent
(always use `ms` units), organize for concision and
rename a few references for clarity.

The only functional change is the addition of the string
"ago" to some of the relative time templating.

Refactor annotation events

Remove state logic from root-thread and place it in corresponding stores:

- deleteNewAndEmptyAnnotations moves to drafts and renamed to deleteNewAndEmptyDrafts
- events.BEFORE_ANNOTATION_CREATED's logic moves to createAnnotation in annotations store
- selection store handles REMOVE_ANNOTIONS action directly

Bump eslint from 6.0.1 to 6.1.0

Simplify root thread generation in top-level UI components