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/
One last note. The messages passed in are coming from xblocks that potentially live in other repos (e.g. ORA2), so there is no guarantee that Studio has a translation for the strings. I think it makes sense for each xblock to own its own translations and to pass in localized versions.