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 view serves two URLs. I added the second usage:

```python
GET /api/courses/{course_id}/content/
GET /api/courses/{course_id}/content/{content_id}/children/
```