WebRTC connection

[okdistribute]

Use the key to start a webrtc connection. Consider porting from webwormhole.

[okdistribute]

I looked at this and did a small prototype using the simple-peer package and it was relatively straightforward.

Here's the related code that could be ported to the backchannel class in the current implementation: https://github.com/inkandswitch/backchannel/blob/webrtc/src/backchannel.tsx#L29-L49

We decided this would be a stretch goal and not necessary for a proof of concept demo. Future versions of the application could upgrade to webrtc when available, it would be ideal in the case where the relay is offline or we can't maintain it anymore. Help wanted!