Self-serve groups

This is part of our publisher and community user stories.

- Users can currently create private groups.
- We're imagining new groups which give new capabilities to publishers (page owners) and communities (groups of users).
- Currently we've imagined that initially Hypothesis admins would create these groups.

This story simply suggests that the groups may have been already created, and our admins might be able to work with / reconfigure an existing group.

The issue names no files, tests, or entry points. Start by reviewing the publisher and community user stories and the current private-group behavior. Before implementation, clarify which existing-group capabilities administrators should be able to create or reconfigure and what the completed behavior should be.

Written by the indexing model from the issue text.