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.
> The placement of this messages seems odd (not where the Delete button would be). I agree with just disabling the Delete button, with a tooltip or something that explains why it is deleted.

Done.