Restricted groups for communities

As a community I want to have a community controlled and branded group that anyone can read, but that only invited members can write to, so that I can limit annotations to my community members and not need to moderate the annotations.

**Note**: this is an **Epic** - a big user story that's too big to go into a sprint, it'll need to be fleshed out and split up into smaller stories before we can start working on it.

Notes:

* Same issue as for [open groups for communities](https://github.com/hypothesis/product-backlog/issues/425): when does the group appear in the groups dropdown menu for non-members?
* Same issues as with [restricted groups for publishers](https://github.com/hypothesis/product-backlog/issues/424).

Start by reviewing the related open-groups issue #425 and restricted-publishers issue #424, then clarify when restricted community groups appear in the groups dropdown. Break this epic into smaller stories with defined behavior and acceptance criteria before implementation can begin.

Written by the indexing model from the issue text.