Make course_groups managed by south
[migration!] Change UNIQUE constraint in course_groups...

...to include group_type.  Adds a migration.
Add 'experiment' as a type of group
Mwahahaha.  It lives!

TODO:
- tests
- hiding of children
- figure out how to move segments lib out of xmodule
- figure out atomicity concerns in segmentation.py
- test nested verticals
- proper staff UI
- pull out un-needed changes to the course_groups model.
  - consider whether we want to merge and migrate that model, or whether
    the new approach here is better, and we should leave the cohorts code
    alone.
- performance impact?
- analytics logging
- docs
...

but it lives!