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.
> Description is optional (at least in the UI). Is this validation necessary? Looks like you want to make sure description is a basestring, but that level of validation is not done for name.

Yes, it is optional on front-end, but we need it to instantiate `UserPartition`. Yes, I think this validation is redundant, because we take care about `description` fields on client side. Removed.