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

[Vincas Dargis]

Ben, could you try my WIP patch for fixing this (and similar) issue:

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

After doing like this:

```
wget -O /tmp/usr.bin.thunderbird /etc/apparmor.d/usr.bin.thunderbird 
https://gitlab.com/Talkless/apparmor-profiles/raw/252347e48bccdcc0e1c070b4d2a5ce68963855e0/ubuntu/18.04/usr.bin.thunderbird

sudo cp /tmp/usr.bin.thunderbird /etc/apparmor.d/

sudo aa-enforce /etc/apparmor.d/usr.bin.thunderbird
```

Thanks!