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

[Vincas Dargis]

Please test my WIP solution:

https://gitlab.com/Talkless/apparmor-profiles/blob/fix-thunderbird-signature/ubuntu/18.04/usr.bin.thunderbird

Biggest danger is the fact that, although it is more secure now, it might deny access to some not yet discovered 
`$HOME/.directory/whatever`, as I got with `.cache/thumbnails/...` case.

Profile was really too permissive, with:

owner @{HOME}/** rw,

meanwhile denying only dot files, but not directories:

deny @{HOME}/.* r,

I have tested on various desktop environments, discovered some .files to be denied to reduce noise. But maybe (probably) 
your $HOME has more to be added for sake of silence in the logs :) .