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/
The idea is that studio_editable provides mixins to make providing custom Studio editors easier. This includes but "studio_view" and "author_view"-- they are both views specifically for Studio that allow editing.

If you have an xmodule, you have both a module and a descriptor. With xblocks, there is only one class.

The descriptor mixin is really just providing pass-throughs to what is defined in the module, because sometimes we only have the descriptor in Studio. There are a whole bunch of these module_attr mixins defined up in one of the superclasses.