Turn media.peerconnection.ice.relay_only to true as defense in depth against WebRTC ICE leaks

From this note:

The "safer" option [to avoid leaks in a WebRTC-enabled scenario] would be dis-allow the usage of the so called host candidates. In the most privacy conserving scenario only use TURN candidates. TURN servers are relays which are meant to help traverse restrictive NATs, but since they are relaying they also effectively hide the publicly routable IP address of your ISP/uplink. Firefox has an about:config parameter called "media.peerconnection.ice.relay_only", which makes Firefox WebRTC stack only connect via TURN.