Refactoring and reformulating caching scheme on topic tree data, to balance between correctness and efficiency.
Merge branch 'release-0.11' of github.com:learningequality/ka-lite into better_topic_caching

Conflicts:
	kalite/main/topicdata.py
	kalite/main/views.py
	kalite/templates/video.html
Merge branch 'release-0.11' of github.com:learningequality/ka-lite into better_topic_caching

Conflicts:
	kalite/main/views.py
Merge branch 'release-0.11' of github.com:learningequality/ka-lite into better_topic_caching
Merge pull request #1101 from bcipolli/better_topic_caching

Refactoring and reformulating caching scheme on topic tree data, to balance between correctness and efficiency.