Make video interface language aware (back-end commands don't yet obey...)

Conflicts:
	kalite/templates/updates/update_videos.html
Fix for #227 - call videoscan on an (async) api request, rather than in a (blocking) view
Make back-end commands locale-aware
Merge branch 'develop' of github.com:learningequality/ka-lite into 1587

Conflicts:
	kalite/main/api_views.py
	kalite/main/views.py
Now show language no matter what, and reload available language packs upon setting
Merge pull request #1597 from bcipolli/1587

Easy language switching from video download page