[DESIGN] Scoping and design work for Public groups

Initial UI work needed:

**v1**
- [x] Publishers: Publisher Groups (Restricted Membership Groups made up of experts, Authorised can write, world can read)
- [x] Hypothes.is: Open Groups (Anyone can join + read, members can write), Public Groups (Invite only, anyone can read, members can write) + Private Groups (Invite only, members can read + write)
- [x] Showing one group as authoritative over the page
- [x] Handling the use case when users lack annotate permissions (e.g disabling adder)
- [x] Showing the number of annotations within a group
- [x] Group activity pages
- [x] Showing groups with different levels of permissions (read-only, write)
- [ ] Requesting to Join a 'controlled' access public group.
- [ ] Basic admin interface building on what we already have for edit group.
- [ ] Rules around authoritative group e.g. if none designated H public channel is the designated authoritative group.

**v2**
- [ ] Group Discovery
- [ ] Joining a public open group.
- [ ] Logic behind the display of group order e.g most active
- [ ] Domain-level activity pages
- [ ] Scaling to multiple private, controlled and public open groups. Search, Filtering, Sorting etc.
- [ ] Exploring more complex/intuitive rules around displaying/order of groups list in group switcher.
- [ ] Looking at how the Post to button works across multiple groups e.g post to multiple groups, or a different domain from current scope.
- [ ] How to show individuals as authoritative within a domain.
- [ ] Robust moderation tool.