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/
@nasthagiri These are very good concerns. Leslie has expressed that we shouldn't address concurrent editing at this point, but that we'll put it on the backlog. Hopefully once we have split we can pass the version number that we are editing to the server and it can reject the update if something has changed.