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.
@polesye Could you pull out ```NoteSectionView``` and put it in its own file ```views/note_section```?