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 
This feels very brittle... anytime I see "parse" and "html", I get nervous!

I suspect that other developers will expect they can change the appearance of the overview page without impacting the API and/or API consumers.