Implement a browser-compatible NAT behaviour discovery

So far we've been lucky that the npm modules we've been using are available through Firefox and Chrome APIs. That's not the case for stun, which we need for legacy/trac#34129 (moved).

It looks like there are some options for packing node modules for use in web applications: https://www.npmjs.com/package/webpack

But we might need to modify our build scripts.

Now it seems like we'd have to implement our own STUN library from scratch using the UDP socket APIs.

It's questionable how worth it this is at the moment.