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 would suggest moving all of the modules that are currently in the server_api package into an api_utils package somewhere outside of djangapps, and make a course_content_api package under djangoapps for the things currently in server_api/courses.