Use Prettier to auto-format .scss files

Merge removal of dropdown-menu-btn.scss

Convert and apply new menu styles, icons to annotation-publish component

- Add `z-index` to menu-content elements to make sure they are rendered
  on top of other sidebar elements
- Bump `z-index` on menu arrow so that it is above `menu-content`

Separate hover areas for main action and submenu toggle

Merge in changes from https://github.com/hypothesis/client/pull/1188

The `coverage/` dir contains generated JS and CSS files, produced when
running tests.

Make `yarn format` and `yarn checkformatting` handle .scss files as well
as JS files, so we can get the benefits of consistent, automated
formatting for our SASS/CSS.

Menu usability improvements

Bump dompurify from 1.0.10 to 1.0.11

Bump preact from 10.0.0-beta.2 to 10.0.0-beta.3

Bump focus-visible from 5.0.0 to 5.0.1

For menu items that have associated submenus, make it clearer that there
are two separate click zones by highlighting only the clickable area for
the main action when hovering that part of the item.

For menu items without submenus, there should be no visual change.

The implementation adds a `.menu-item__action` element that wraps the
content of the menu item excluding the submenu toggle.

The CSS has been re-organized to group together all of the rules that
affect a specific element. This makes it easier to verify that there are
no states which visually conflict for a particular element.

Bump eslint-plugin-react from 7.13.0 to 7.14.1

Refactor clearSelection out of sidebar-content and into the store

Bumps [preact](https://github.com/developit/preact) from 10.0.0-beta.2 to 10.0.0-beta.3.
- [Release notes](https://github.com/developit/preact/releases)
- [Commits](https://github.com/developit/preact/compare/10.0.0-beta.2...10.0.0-beta.3)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Bumps [eslint-plugin-react](https://github.com/yannickcr/eslint-plugin-react) from 7.13.0 to 7.14.1.
- [Release notes](https://github.com/yannickcr/eslint-plugin-react/releases)
- [Changelog](https://github.com/yannickcr/eslint-plugin-react/blob/master/CHANGELOG.md)
- [Commits](https://github.com/yannickcr/eslint-plugin-react/compare/v7.13.0...v7.14.1)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Bumps [focus-visible](https://github.com/WICG/focus-visible) from 5.0.0 to 5.0.1.
- [Release notes](https://github.com/WICG/focus-visible/releases)
- [Commits](https://github.com/WICG/focus-visible/compare/v5.0.0...v5.0.1)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Bumps [dompurify](https://github.com/cure53/DOMPurify) from 1.0.10 to 1.0.11.
- [Release notes](https://github.com/cure53/DOMPurify/releases)
- [Commits](https://github.com/cure53/DOMPurify/compare/1.0.10...1.0.11)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Use clearSelection from the store instead of from sidebar-content.

- This clears the selected annotations, groups, direct-linked
  group error, and resets the selected tab to annotations if it's
  currently set to orphans.
- Change the action of clearSelectedAnnotations to CLEAR_SELECTED_ANNOTATIONS
  so CLEAR_SELECTION can be used as the action for clearSelection.
- Add tests in index-test over the global store since this triggers an
  action in multiple store modules.

Click and mousedown events on a menu are no longer propagated upwards.
Therefore the `document.body`-level event listeners that cause a menu to
close when clicking outside of the menu need to use capture rather than
bubble-phase listeners.

 - Do not close the menu if the user presses the mouse in the lower
   right corner of the toggle button _just_ where the notch/arrow at the
   top of the menu content appears
 - Do now show outline when pressing mouse down on a menu item. An
   outline will still be shown if the menu has keyboard focus
 - Close menu when an item is activated with Enter or Space keys

Add "Copy link" action to groups menu

Feedback on the first iteration was that "Copy link" was ambiguous. Add
the term "invite" or "activity" to the label to indicate what the link
is for, depending on whether it is a group that the user can join or
not.

Replace references to old groups menu styles in new component

Update icon for "New private group" menu item

Update to latest icon from https://hypothicons.netlify.com.

Add an action to provide a convenient way to copy the sharing link
directly from the groups menu.

Depending on whether the group is private or open and whether the
receiving user is a member or not, the link may present the user with a
Join page or the activity page for the group. To simplify the client logic, the
text just says "Copy link".

I evaluated popular npm packages for this purpose (eg. [1]) but encountered
issues where they were not transpiled for use in older browsers or
included workarounds for browsers we no longer support (eg. IE 9).

If we do decide to use an existing package in future, we can keep this
utility module as a wrapper around it.

[1] https://github.com/sindresorhus/copy-text-to-clipboard

This replaces some references to the styles for the old groups menu in
the new implementation. The menu label is now slightly darker as it
uses the same grey as the menu items. Otherwise there should be no
noticeable visual changes.

The CSS itself has been simplified compared to the previous
implementation.