Add HTML5 player support for long videos

[givik]

[gigagvenetadze]

It's will be good to make so 👍

[Joshfindit]

What's required for this, and what does it consider a long video?

I've successfully used this for HTML5 videos that have been processed with ffmpeg's -movflags faststart and it does work. Mostly. If we catch it just right it will be synched to within 50ms, but it does struggle to do that and will sometimes 'slip' while watching. Our fix so far has been both exit the room, rejoin, reload, skip to where we were, and continue. Inconvenient, but not a deal-breaker.