TNL-762: Add course structure view.

**sandbox:** http://polesye.m.sandbox.edx.org/courses/edX/DemoX/Demo_Course/edxnotes/

@explorerleslie, @clrux, @jmclaus, @olmar please review.
> Could you pull out NoteSectionView and put it in its own file views/note_section?

We'll use it just in this file. So, I do not see any reasons do move it into a separate file and add additional request.