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.
@tymofij  Do we need to check for the result of `unit.get_parent()` before call `section.get_parent()`?