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/
By the way, is the plan to eventually remove the container-specific views?  It's unclear why they have specific views.  Instead, we can make use of a property on the xblock to indicate whether it's a container, no?