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 `NoPathToItem` was raised by `path_to_location`. Do we still need to catch it here? The same question about `ValueError`.