Courses API

This is a subset of #6109. The goal here is to trim the code down to exactly what will be needed by Insights and get it merged soon. We will revisit the other APIs soon after so that the edx-solutions fork isn't too divergent.

@mulby @mattdrayer @cpennington @ormsbee @dsjen @smagoun @shnayder @mikigoyal 
@clintonb I was waiting for @ormsbee to chime in on this thread.

At the barest of minimums I think we should include a version number in the URL for this API.