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. 
@chrisndodge Good question. This will be the new convention for everyone to follow once this feature branch lands late December/early January. The goal is to remove all of the ```sys.path``` hacks from LMS/CMS by instead using regular packages. In particular, the current hack means that it is not possible to register entry points for Django apps in the common directory. The plan is to migrate all of these Django apps to the new openedx package over time.