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/
Why is this called `StudioEditableDescriptor` if it's dealing with the `author_view`.  From my reading of the code, the `author_view` supports the preview of the xModule in studio.  Since it doesn't necessarily supporting Editing, should this be renamed?  
Instead, it's the `studio_view`, by definition, that actually supports Editing.