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.
> This method isn't being used. Perhaps it should be when generating a new ID above? If it is not needed at all, delete it.

I've added additional method to make sure that newly generated ID is unique in the list of currently used IDs. See [course.py#L904](https://github.com/edx/edx-platform/blob/anton/edit-group-configurations/cms/djangoapps/contentstore/views/course.py#L904)