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.
> It took me awhile, but I think I finally understand that self.configuration is a Python dict (obtained from deserializing a JSON string). I think it would be clearer to make self.configuration an instance of UserPartition, and then rely on UserPartition's to_json and from_json methods.

Fixed.