MIT: CCX. Implement Custom Courses for Edx.

This feature provides the ability to designate a "coach" who can create customized runs of an existing course, invite students to participate, and manage students through the run of the course.

In this squashed commit we implement the initial scifi, add the 'POC Coach' course role, refine the scifi, add migrations for models, create POCs, enforce POC Coach role, provide panels for Coach Dashboard, set up rudimentary display of course outline, add and remove units, show/hide all units, and save schedule changes, set dates when adding units, edit dates on units already added and provide some tests.

We also provide mechanisms for invitation and enrollment in a POC (to become CCX) and control the display of blocks to students in a POC.