UI for setting custom STUN server. Include core-elements in UI.

ran grunt test.

Allowing users to set STUN servers introduces a new bug: users can be stuck with poorly configured SocksToRtc/RtcToNet instances which require restarting uProxy to fix. More here: https://github.com/freedomjs/freedom/issues/163
these configs seem very similar to those of rtcToNet, can you make it a variable and reuse it?