feat: use DASH support using dash.js

[pimlie]

I gave dash.js another look, this PR adds dashboard using dash.js. It's probably best to either merge this pr or #179, but not both.

• This is something I still don't like about DASH.js, it makes a partial content request for all resouces in the dash playlist when loading the player. It might be possible to tune this behaviour, but I haven't found a way to do that so please let me know if you do :)

• The dash player loads whenever the video element scrolls into view, not just when you click play. Let me know if delayed player initialization is a hard requirement

This is again not a breaking change. USE_HLS should still work as previously. If you enable both then USE_DASH has preference