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 confused why from_json is called. self.configuration is initialized with GroupConfiguration.parse(json_string). So what is self.configuration?

I think a big part of my confusion is that it doesn't appear you are using the UserPartition class. I would expect you to, since it has to_json and from_json methods.