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

[Vincas Dargis]

Thunderbird fails to read signature even on complain mode, because AFAIK complain mode does still mediate `deny` clauses:

sudo sysdig "fd.name contains signature"
271543 15:19:14.386997890 6 thunderbird (3680) < open fd=-13(EACCES) name=/home/vincas/.signature-foo at example.com 
flags=1(O_RDONLY) mode=0

I guess there will be something in `abstractions/private-files` or similar, gonna check all deny rules.