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 
It sounds useful enough that we should just include `format` in all responses.