[pkg-apparmor] Bug#882048: apparmor should let thunderbird use signatures from files

[Vincas Dargis]

After seeing https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=882218 I changed my mind. It's too complicated :( .

If Thunderbird profile path would be a AppArmor parser variable, that could be (somehow?) updated by 
local/usr.bin.thunderbird, these kind of customizations could be dealt with by user.

Currently I guess I have to vote for keeping it disabled by default. It would be nice to have encouragement to used it 
in README though, to reach as much as possible of testers/users.