Add audio/video controls to device request page

[wolfgang42]

So you can turn off your audio/video before joining, to avoid e.g. giving everyone horrible background noise.

WIP, known issues:

• Right now the controls are just plunked in the form. I tried briefly to move them next to the dropdowns, but ran into CSS issues and gave up for the night.
• I haven’t tested this, but I suspect if you switch to a different device things will get confused and the local state will go out of sync with the underlying stream device.

I had to do some refactoring; the commit messages should explain what’s going on.

[wolfgang42]

Yeah, I confirmed that changing the device causes the stream state to go out of sync with the values in the Local atom. But I’m having trouble figuring out what the right fix for this is; it seems like right now the stream setup code is a bit scattered. I’m not very familiar with Recoil, so this might be a naïve question, but: is there a reason there isn’t a “LocalStream” atom that has {audioDeviceId, videoDeviceId, audioEnabled, videoEnabled} plus an effect to convert that into the appropriate calls to lib/mesh/stream.ts?