Adding "video_id" and "exercise_id" to the topics.json files.  Makes the code read more easily (future)
Merge branch 'release-0.11' of github.com:learningequality/ka-lite into pre_dubbed_video_support
Use video_id for all VideoLog saving & querying, youtube_id for all VideoFile and video showing.  Requires schema changes, code changes, and variable renaming to get the semantics right.
Sorry for the huge commit, but most everything has to come in, in order to work.
Making Id based on type, separating id and slug, removing readable_id.

ids are anything unique.  slugs are purely for url-building.  node_cache stores by ID.
Migrating to id/slug schema, with NODE_CACHE storing as ids, not slugs.
Beginnings of dubbed video support by i18n API get_video_id(youtube_id)
fixing string wrapping errors, linting to make text searches for such errors easier.

Conflicts:
	kalite/updates/management/commands/videodownload.py
Merge branch 'release-0.11' of github.com:learningequality/ka-lite into pre_dubbed_video_support

Conflicts:
	kalite/updates/management/commands/videodownload.py
Merge branch '999' of github.com:bcipolli/ka-lite into pre_dubbed_video_support
Merge branch 'release-0.11' of github.com:learningequality/ka-lite into pre_dubbed_video_support
Prevent the cronserver from being launched on the central server.
Merge pull request #1100 from bcipolli/no_central_cron

Prevent the cronserver from being launched on the central server.
Merge pull request #1091 from bcipolli/pre_dubbed_video_support

Pre dubbed video support: migrating schema, scrubbing code.
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