Add support for courseware partitioning based on cohort.

@andy-armstrong @cahrens 

suspect i'm going to need to wire something into stevedore - @andy-armstrong ?

please take heed of the notes in the first commit message - there's a database migration needed. 
this package wasn't yet under migrations.  the first migration "converts" the package, defining the CourseUserGroup model already in play, and the second migration adds the model that's new in this change.