A user reported that the TorBirdy status bar is updated if they toggle
the state of themes (enable/disable). This commit fixes that issue by
restricting the status bar update to TorBirdy.

In 25113d08, we added support for displaying information about TorBirdy
after it was installed and running for the first time. It was suggested
that this may confuse the users so it was disabled in dfb1dc8c.  This
commit removes the obsolete code instead of just disabling the
preference.

Assume that a user has dirmngr running and configured with `use-tor' in
dirmngr.conf and set the relevant keyserver settings (similar to what
Tails has been doing); otherwise the keyserver lookup will fail-safe.
This should work for the daemon and Tor Browser since as per
gnupg/dirmngr/dns-stuff.c, it tries both 9050 and 9150. This is probably
better than the older setting because a) it actually works now, b) it
will still fail-safe in case Tor is not running.

Also set the Content-Language to "en-US" to prevent leaking the user's
default dictionary. Reported in https://bugs.torproject.org/22484 and
discussed in Section 4 of RFC 3282.  Thunderbird bug:
https://bugzilla.mozilla.org/show_bug.cgi?id=1370217

- Bump version to 0.2.4
- Set minVersion to 52.0 and maxVersion to 59.*

- Update import-translations.sh (tor-launcher script)
- Update and add existing translations from Transifex to TorBirdy

See the ticket above for more information on what encrypted email
headers are and how they work. This setting can be disabled using
TorBirdy's preferences.