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/
If we notify that the save has completed, we will have to send an error state as well. If there is an error, it will be shown by our general "Studio is having trouble saving your work" handler, replacing the saving notification. If we close the saving notification (which is what this callback does), I think it would close the error message.

@andy-armstrong, what do you think? I'm content with it as-is, but that is assuming knowledge of what the runtime is doing.