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.
> Is configuration_json used anywhere? What is it a json representation of (given json_string mentioned above)?
> 
> Also, group_configuration is mentioned, but I don't see it in the code.

Sorry, I think it's due to conflicts during rebase. Removed.