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 
I'm a little nervous about this endpoint being too closely scoped to the current use case. I get wary of anything that is specific to particular types of content. For example, I might expect this to return ORA2 problems in course, however, I suspect it does not (granted I don't know much about course content).

I'm also not familiar with the "assignment" terminology... is that from studio? Is it something we want to continue using in a generic XBlock world?

Is there some reason the client couldn't do this work, just grab the content and then filter it down to whatever subset it actually cares about?