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.
> As noted in my previous review, I was hoping that self.configuration would be an instance of UserPartition. Why did you decide not to do this?

`self.configuration` used for internal purposes. It is used to prepare some required data (for example, ids) before  we can instantiate UserPartition. Also, it gives us possibility to provide user-friendly error messages. Further in the code we're working with instance of UserPartition that is returned by [get_user_partition](https://github.com/edx/edx-platform/blob/anton/edit-group-configurations/cms/djangoapps/contentstore/views/course.py#L949): [group_configurations_detail_handler](https://github.com/edx/edx-platform/blob/anton/edit-group-configurations/cms/djangoapps/contentstore/views/course.py#L1028), [group_configurations_list_handler](https://github.com/edx/edx-platform/blob/anton/edit-group-configurations/cms/djangoapps/contentstore/views/course.py#L986)