Add possibility to remove group configurations.

**Ticket**: [BLD-1049](https://openedx.atlassian.net/browse/BLD-1049)

**Acceptance criteria:**

* If any content experiments are using the Group Configuration, delete should be unavailable (provide some text inline that says that groups are in use and cannot be deleted).
* If no content experiments are using the Group Configuration, a delete link should appear that deletes the context experiment. * 
* This is irreversible, so throw a similar “Delete this Group Configuration?” modal message to let the user confirm the delete. 

**sandbox:** http://studio.polesye.m.sandbox.edx.org/

@andy-armstrong , @cahrens , @frrrances , @mhoeber please review.
> Why is it a POST instead of a DELETE?

[emulateHTTP](http://backbonejs.org/#Sync-emulateHTTP) is enabled in [main.coffee](https://github.com/edx/edx-platform/blob/master/cms/static/coffee/src/main.coffee#L15), so setting this option will fake PUT, PATCH and DELETE requests with a HTTP POST, setting the X-HTTP-Method-Override header with the true method.