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

[Vincas Dargis]

Looks like the culprit is this line in usr.bin.thunderbird [0]:

```
deny @{HOME}/.* r,
```

I am not a maintainer of Thunderbird, but I _guess_ that the story is like this:

1. AppArmor profiles denies everything what's not allowed by default.

2. Thunderbird profile has only some dot-direcories allowed to be read, like this rule: `owner 
@{HOME}/.{icedove,thunderbird}/ rw,` [1].

3. In the result, there are possibly a lot of dot-files and directories in user $HOME that's will not be accessible for 
Thunderbird process.

4. Opening a File dialog to select file to be attached, produces bunch of DENIED messages in log, when user browses it's 
$HOME, which contains dot-files and directories. I have experienced this myself, as for some reason file select dialog 
tries to read files being displayed (probably for create/modify dates?). To avoid these noisy DENIED messages, someone 
have put `deny @{HOME}/.* r,` rule to silence it. This is my speculation.

This produces sort of bug that rule `/tmp/.X[0-9]*-lock r,` [2] is not actually working, as `deny` one should AFAIK 
override this allow one. But that's for another topic.

Anyway, I believe change to allow Thunderbird to read arbitrary dot-files or directories will not gonna happen, so 
solution would be to:

1. Rename your `/home/ben/.signature-ben at transient.nz` file so that it would not start with dot.
2. Move `.signature-ben at transient.nz` into other sub-directory, like `/home/ben/Documents/.signature-ben at transient.nz`.

It would be useful if AppArmor would have possibility to override `deny` in `/etc/apparmor.d/local/usr.bin.thunderbird`, 
something like this:

`override owner @{HOME}/.signature-ben at transient.nz r,`

So user could overcome some `deny` rules... But I believe we do not have that functionality.


[0] https://anonscm.debian.org/cgit/pkg-mozilla/thunderbird.git/tree/debian/apparmor/usr.bin.thunderbird#n118
[1] https://anonscm.debian.org/cgit/pkg-mozilla/thunderbird.git/tree/debian/apparmor/usr.bin.thunderbird#n163
[2] https://anonscm.debian.org/cgit/pkg-mozilla/thunderbird.git/tree/debian/apparmor/usr.bin.thunderbird#n90