Fixes for MOB-836 and MOB-766.

https://edx-wiki.atlassian.net/browse/MOB-836
https://edx-wiki.atlassian.net/browse/MOB-766

*Note:* I've been meaning to update the unit tests for these changes.  But wanted to start the review process as soon as possible.  I'll try to work on the tests later tonight.

@ormsbee Can you take a look through this? 
@waheedahmed I would appreciate a review from your side since you are familiar with the video code.
@adampalay FYI.
Is there any reason to traverse down this portion of the tree at all then? Would it simplify the code if we did something similar to the access check on line 89 and simply `continue` for any block that has `hide_from_toc=True`?