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/
Once again, the lack of transaction protection worries me.  
What happens if multiple authors edit the experiments at once?
What happens if the client-side data is out of synch from the server's?

Even if we don't worry about the former, we should be robust against the latter.  The latter can legitimately happen if, for example (1) a user starts to edit an outdated webpage that they didn't refresh or (2) there's a server error and the client-side is updated but the server isn't.  For such cases, we should consider having the server validate the data before updating it.  

Not sure if we can address this at this time, but something we should put in our backlog.