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.
We need to check the `hide_from_toc` field on all ancestors of the block, not on just the block itself.  I had chosen to add the check here since the ancestors have been computed by this point.  

However, it's possible we can also add the check as it computes the ancestors in the `path` method.