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. 
it's not possible to create more than one, this is currently tested here https://github.com/edx/edx-platform/pull/5949/files#diff-240cc9f3333e5096b19c92ae35f0548eR615