Open groups for communities

As a community I want to have a group or groups that anyone can read or write but that I can moderate, so that I can separate my community's distinct conversations into their own spaces and prevent abuse.

**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.

Problems:

* Community groups aren't restricted to a publisher's domain like publisher groups
are. When does the open community group appear in the groups dropdown menu, for
which users?


Currently:

* The **Public** group appears in the sidebar always, for everyone (except on
eLife's site).
* All the **private groups** that a user has joined appear in the sidebar all
the time for that user. Only user's who've joined a private group see it in
their sidebar.
* eLife's **third-party group** appears on the sidebar all the time only on
eLife's site. (And on eLife's site you're either logged out or you're logged in
and are a member of the eLife third-party group, other users can't be logged in
on eLife's site).
* **Publisher open groups and restricted groups** appear in the sidebar all the
time, for all users, but only on the publisher's domain.
* **Community open groups and restricted groups** should appear in the sidebar
when... ?

No files, tests, or entry points are identified. Start by splitting this epic into smaller stories for community-group visibility, read/write access, and moderation, then define which users see each group in the sidebar and the acceptance criteria for each story.

Written by the indexing model from the issue text.