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. 
@jimabramson and @cahrens the medium term requirements (e.g. FY2015) will have one cohort group mapped to one content group. Because of this, I'd like to focus just on this use case for right now, instead of trying to make this work for use cases much farther down the line.