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. 
yes autogenerated (then slightly modified to placate the quality test).  worth scanning if you're interested, but we wouldn't normally make substantial changes to this.