Update group menu styles

This enables consumers to use `&&` or `||` conditionals to compute the
`icon` prop.

Convert `<search-input>` component to Preact

 - {show, hide} => {renders, doesn't render} to clarify that we're
   checking whether an element is rendered at all, as opposed to merely
   visually hidden
 - Replace ambiguous GitHub issue reference with URL

This makes it easier to see when a submenu item is hovered.

 - Use the string "blank" instead of `null` as a more explicit indicator
   that a menu item should show a blank space for an icon
 - Rename two variables to indicate their boolean-ness

The icon is from the Hypothicons set at https://hypothicons.netlify.com

Following a design review, adjust the styling of menus and in particular the
groups menu to better match the mocks [1] and create better separation between
different elements.

- Add a 10px margin below section headers
- Add a subtle border at bottom of expanded submenus
- Remove red hover color for menu items
- Make submenu items slightly lighter and darken them on hover
- Reduce font size to match top bar and reduce font weight for submenu items
- Increase submenu toggle caret size slightly (10 => 12px) and darken
  the caret on hover, consistent with the effect when submenu items are
  hovered.
- Avoid hyphenating messages in the footer of group list submenus

[1] https://sketch.cloud/s/D0OMk/a/EbnxQA/play

Bump focus-visible from 4.1.5 to 5.0.0

Bump @octokit/rest from 16.28.0 to 16.28.2

 - Remove `@import` commands which duplicated SASS imports elsewhere in
   the bundle
 - Remove `:focus` selector which had no effect
 - Remove an unnecessary `@at-root` block which was used to contain a
   variable that was only used in one place and replace that usage with
   a constant instead

 - Rename `simple-search.scss` and the CSS classes it defines to match
   the name of the module which defines the component structure and
   logic
 - Remove reference to `simple-search-inactive` CSS class which did not
   exist

Merge pull request #1176 from hypothesis/dependabot/npm_and_yarn/babel-plugin-transform-async-to-promises-0.8.12

Bump babel-plugin-transform-async-to-promises from 0.8.11 to 0.8.12

Bump enzyme-adapter-preact-pure from 1.13.4 to 2.0.0

This test happened to pass before, but failed after an adapter upgrade.
All external state changes should be wrapped in `act` to flush state
updates and effects and followed by `wrapper.update` to update the
Enzyme adapter.

Bumps [@octokit/rest](https://github.com/octokit/rest.js) from 16.28.0 to 16.28.2.
- [Release notes](https://github.com/octokit/rest.js/releases)
- [Commits](https://github.com/octokit/rest.js/compare/v16.28.0...v16.28.2)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Bumps [enzyme-adapter-preact-pure](https://github.com/preactjs/enzyme-adapter-preact-pure) from 1.13.4 to 2.0.0.
- [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/v1.13.4...v2.0.0)
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 0.8.11 to 0.8.12.
- [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/v0.8.11...v0.8.12)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Bumps [focus-visible](https://github.com/WICG/focus-visible) from 4.1.5 to 5.0.0.
- [Release notes](https://github.com/WICG/focus-visible/releases)
- [Commits](https://github.com/WICG/focus-visible/compare/v4.1.5...v5.0.0)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Convert loading spinner to Preact

Remove passing of search

Previously a search object was built inside the hypothesis-app
component and passed down to its child components. Now, just use
the store.getState().filterQuery directly. This avoids unnecessarily
passing things between components and a layer of indirection between
the search object and the value in the store. Rename searchQuery to
filterQuery.

Simplify groups service `load` method using async/await

As per previous commit, it is currently possible for client
configuration to specify both an annotation to focus and an additional
group to include.

Move sidebar-content funcs to search-status-bar

Convert `help-link` component to preact

- filterMatchCount
- areAllAnnotationsVisible
- Remove passing of total-count as it's not used.

- Add tests

Replace help icon with lighter variant

- Add `height` rule to `top-bar__btn` to help with alignment

Implement new groups menu design built on groups menu

Also mock the `MenuItem` component manually so that any problems with
that component won't cause a cascade of failures in `GroupListItem`.