Disable WebRTC

Visiting http://net.ipcalf.com/ with Tails 0.22~rc1 displays the local IP address. Pretty scary…
TBB has disable that option at compile time and we should do the same.

https://trac.torproject.org/projects/tor/ticket/8178

Feature Branch: bugfix/6468-disable-webrtc

Related issues

Related to #6370 (closed)
Related to #6474 (closed)

Original created by @sajolida on 6468 (Redmine)

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