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 @cahrens I originally tried using ```always``` in a number of these situations, but as you say it clashed with the AJAX error handling. I would prefer that our code explicitly handle both success and failure conditions, because right now the fallback error message is not very clear (it always says that it was a problem saving when that is often not what is going on). However, for now I would prefer to leave the logic as is, since it is consistent with the rest of the code.