Ability to change the associated group configuration for a split test

STUD-1658

My sandbox has the latest version of this PR.
http://studio.cahrens.m.sandbox.edx.org/
The problem is that the delete action is for any xblock shown on the page. So if you delete a video inside an active group, and you have no inactive groups, then this will fire.

I have a fix for this that I'll push up shortly. The notification already passes the locator, so I just verify that it matches the split_test's own locator.