Only count videos in topic calculation; compute (relevant) video urls only for exercise, video, and "bottom level" topics
"bottom-level" topics should not say they contain topics--they don't!
in a rush, it's Thanksgiving.
Merge pull request #1169 from bcipolli/efficient_url_computations

Efficient url computations
Merge branch 'release-0.11.1' of github.com:learningequality/ka-lite into determine_ip_addresses

Conflicts:
	kalite/main/views.py