Format gulpfile.js and files in scripts/ using Prettier

Eliminate an inconsistency in the client codebase that most files are
formatted with Prettier but files in scripts/ and gulpfile.js were not.

Show focus indicator on various UI elements if they have keyboard focus

Bump express from 4.16.4 to 4.17.0

Bump prettier from 1.17.0 to 1.17.1

Bump @octokit/rest from 16.25.4 to 16.26.0

Bump codecov from 3.4.0 to 3.5.0

Bump core-js from 2.6.5 to 2.6.6

Bumps [prettier](https://github.com/prettier/prettier) from 1.17.0 to 1.17.1.
- [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/1.17.0...1.17.1)
Signed-off-by: dependabot[bot] <support@dependabot.com>

Bumps [express](https://github.com/expressjs/express) from 4.16.4 to 4.17.0.
- [Release notes](https://github.com/expressjs/express/releases)
- [Changelog](https://github.com/expressjs/express/blob/master/History.md)
- [Commits](https://github.com/expressjs/express/compare/4.16.4...4.17.0)
Signed-off-by: dependabot[bot] <support@dependabot.com>

Bumps [@octokit/rest](https://github.com/octokit/rest.js) from 16.25.4 to 16.26.0.
- [Release notes](https://github.com/octokit/rest.js/releases)
- [Commits](https://github.com/octokit/rest.js/compare/v16.25.4...v16.26.0)
Signed-off-by: dependabot[bot] <support@dependabot.com>

Bumps [core-js](https://github.com/zloirock/core-js) from 2.6.5 to 2.6.6.
- [Release notes](https://github.com/zloirock/core-js/releases)
- [Changelog](https://github.com/zloirock/core-js/blob/master/CHANGELOG.md)
- [Commits](https://github.com/zloirock/core-js/compare/v2.6.5...v2.6.6)
Signed-off-by: dependabot[bot] <support@dependabot.com>

Bumps [codecov](https://github.com/codecov/codecov-node) from 3.4.0 to 3.5.0.
- [Release notes](https://github.com/codecov/codecov-node/releases)
- [Commits](https://github.com/codecov/codecov-node/commits)
Signed-off-by: dependabot[bot] <support@dependabot.com>

Enable a focus indicator for top bar buttons and the selected tab when
using keyboard input

The :focus-visible pseudo-class [1] enables showing or suppressing focus
outlines depending on the input modality, eg. hiding focus outlines by
default but enabling them if the user is using a keyboard or otherwise
indicates to their browser that they need a focus indicator.

It is not widely available natively yet though. Add the "official" JS
polyfill for it and a mixin that uses it to suppress an element's
outline when the user agent deems it is not required (typically when
using mouse input).

[1] https://developer.mozilla.org/en-US/docs/Web/CSS/:focus-visible

Implement new menu component and add icons for it

Incorporate build fix from https://github.com/hypothesis/client/pull/1127

Adjust the vertical offset of the content to match our previous dropdown
menus, which looked better.

This provides more flexibility as it can be used to set color as well as
size and it is also more consistent with how everything else is styled in the
application.

Replace the existing use of the `size` prop and tweak menu item icon
sizes.

Fix/simplify manifest rebuild when running "gulp watch"

On my system "gulp watch" was failing to generate the client's entry
point script `build/boot.js` correctly. This may have happened after
recent upgrades to gulp or Node.

The issue was that `generateManifest` was run after "gulp
watch" started, before build/scripts/boot.bundle.js was generated,
producing an empty output file in build/boot.js. It was not re-run later
after build/scripts/boot.bundle.js was generated.

After some experimentation, removing use of `gulp-batch` in the
`watch-manifest` task resolved the issue. I haven't verified whether the
node upgrade or a gulp upgrade was the root cause. Since gulp-batch was
non-essential, I've just removed it to simplify things and reduce the
risk of similar issues happening in future.

 - Add a clearer comment to explain what the global event listener
   effect is for
 - Add comment explaining what `ignoreNextClick` flag is for
 - Remove a test for the `ignoreNextClick` flag when a "click" event
   happens _outside_ of the menu toggle button. This _should_ close
   the menu
 - Use CSS classes to set menu alignment
 - Add some additional tests

The implementation is missing. It will be added in a follow-up PR.

`map` returned its argument but never called its callback!

Following design changes to the menus that will use this component, this
option is no longer needed.

Fix extra underlined space in "log in" link

Add new icons for the new menu component and the new groups menu. Also
add a `size` prop to the `SvgIcon` component so that icons can be sized
appropriately for usage in a given context.

Implement a new dropdown menu component that will replace all drop-down
menus in the sidebar and is also intended to be used in the LMS activity
views in future.

The menu consists of several components:

 - `Menu` is the dropdown menu itself, with a toggle button and menu
   content
 - `MenuSection` is a group of menu items with an optional heading
 - `MenuItem` is a menu item with a label, optional icon and optional
   submenu with additional actions. The submenu is going to be used for
   auxilliary links and actions related to groups in the groups menu

Upgrade node-sass and fsevents for Node v12 compatibility

Refactor groups service's `load` method

 - Capitalize "API" consistently
 - Remove a couple of comments that repeated the code below

Import fix for a warning caused by a "quietly" failing test.

As pointed out during code review, mixing the terms "selected" and
"direct-linked" to refer to direct-linked annotations and groups is
potentially confusing. Use the term direct-linked consistently.

Fix a silently failing test

The `HypothesisAppController#login()` method returns a Promise and calls
`fakeStore.clearGroups` asynchronously. This means we need to wait for
the result before checking the spy, and return the result in the test
case so Mocha knows when the test has finished.

Clarify what the method does and add some comments to explain to the
reader the high-level steps that it goes through.