Edit group configurations.

Ticket: [BLD-1067](https://openedx.atlassian.net/browse/BLD-1067)

**Acceptance criteria:**
* Edit link from Group Configurations page for each group configuration.
* Do not surface group information nor allow groups to be edited (will be a separate story). 

**sandbox:** http://studio.polesye.m.sandbox.edx.org/

@andy-armstrong , @frrrances , @cahrens , @explorerleslie, @mhoeber  please review.
> I am nervous about using a random number. There is always the chance, although obviously remote, of a conflict. I posted a question to the LMS room to see if anyone has a better idea.

I think the chance is too small. We did so because IDs should be unique per course and we're using ID's with length 12. In this case, the chance that duplication occurs is about `1/10^12 = 0,000000000001`.
What do you think?