Security fixes
Upgrade Tor Browser to 7.5.6 (MFSA 2018-17; Closes: #15683).
Upgrade Enigmail to 2.0.7 (partly fixes #15602 aka. EFAIL).
Upgrade libgcrypt to 1.7.6-2+deb9u3 (DSA-4231-1).
Upgrade perl to 5.24.1-3+deb9u4 (DSA-4226-1).
Bugfixes
Thunderbird: fix importing public OpenPGP keys from email attachments
(Closes: #15610).
Make the Unsafe Browser home page translatable again (Closes: #15461).
Minor improvements
Don't display the "Know your rights" message on Thunderbird first run.
Move Thunderbird's default userChrome.css to /etc/thunderbird, just like
we do for Tor Browser, for easier upgrade handling.