Disable WebRTC (Closes: #6468).

On the long run, we want to do this at build time, just like the TorBrowser.
However, fiddling with the build options two days before a Tails release seems
crazy, so let's disable this with a pref for now.

Successfully tested on http://mozilla.github.io/webrtc-landing/ and
http://net.ipcalf.com/.