Add react moderation banner component

Break out `annotation-user` component from `annotation-header`

Rename group-list-v2 => group-list

Now that the old group list/menu has been removed, rename the new
version so that the file name matches the component name.

Always use main URL from main frame when searching for groups

Remove old dropdown menu implementation

Remove old groups menu and "community_groups" feature flag checks

This was used by the old dropdown menus.

The corresponding JS component was removed as part of the recent
redesign of the groups menu.

The new groups list implementation is still in `groups-list-v2.js`
rather than renaming the file to avoid the diff being too confusing.
Renaming it to remove the "-v2" suffix will happen in a follow-up
commit.

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`

Previously the groups service used the first HTTP(S) URL returned by
`store.searchUris()` to get the URL used to search for associated groups. In
practice, the URIs from the main frame always appeared first in this
list, and the main URL from that frame was the first entry within that sublist.
However, the `store.searchUris` function doesn't provide any guarantee of this
and future changes could break this invariant.

This commit adds a `store.mainFrame()` function which returns the main
frame, the one with no `id`, and modifies the groups service to use the
main URI for that frame.

A change in behavior here is that if the top-level frame was a non-HTTPS
frame but embedded an HTTPS frame, then the group search would have used
the URL from the HTTPS frame. Now it will use the non-HTTPS URL from the
top-level frame.

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