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. 
Just a drive-by question. Can you explain this new naming format of openedx.core.djangoapps.....? Are other teams supposed to adopt the same?