Full restructure and rewrite to accommodate for new features

[april83c]

To accommodate for new features such as osuplace/templateManager#62 that would require additional serverside components, and to generally make Broadcaster easier to work with, I think we should do a full rewrite and restructure to

• make it more modular
• use modern web frameworks instead of using raw HTML/CSS/JS like we are now
• generally make the code cleaner

We could also consider using a more performant language such as Rust. This would possibly make it actually viable to have the polling API.

[LittleEndu]

• imo the more important one is Investigate responseType: 'stream' for websocket replacement osuplace/templateManager#53

because we can't rely on every website allowing websocket connections, and the only thing that can bypass it is GMxhr. Polling should be the last last resort.