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. 
I wouldn't say there's a plan, but there's been discussion that we need one. :-) Seriously, we'll work out a way to communicate this soon.