TNL-762: Add course structure view.

**sandbox:** http://polesye.m.sandbox.edx.org/courses/edX/DemoX/Demo_Course/edxnotes/

@explorerleslie, @clrux, @jmclaus  please review.
@tymofij would it be better to move the logic that finds chapter, section into a separate function and reuse it here and in edxnotes?