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 
@mattdrayer and I were discussing this last week. It's a good idea which, with the removal of so much code, is essentially where we are. It's just a matter of fixing namespaces. I'll look into organizing the packages better.