Bug#971524: ITP: briar-gtk -- Desktop and mobile client for Briar p2p messaging

[Nico Alt]

I posted some updates to the packaging of briar-headless in [0] which is 
currently blocking this ITP.

tl;dr: Plans to get into next Debian stable release were given up and 
The Briar Project will now focus on setting up their own APT repository [1].

[0] https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=971526#10
[1] https://code.briarproject.org/briar/briar-debian/-/issues/3