Online video cleanup

This depends on #769, diff can be viewed here: https://github.com/bcipolli/ka-lite/compare/issue234...online_video_cleanup

Support for streaming online videos was checked in, but I found a few bugs in the interface after realizing I hadn't tested all scenarios.

Here are all the scenarios:
* Whether online or not (online, offline)
* Whether the video exists on disk (yes, no)
* Whether backup_video_source is defined (yes, no)
* What page you're on (topic page w/ no videos, topic page w/ videos, video page).

A lot to get right.  This PR gets them all right :)

Testing:
See above.  I did them all.