dev.gnupg.org: Add informative header to all outgoing email.

* src/applications/metamta/replyhandler/PhabricatorMailTarget.php
(PhabricatorMailTarget::willSendMail): Add informative header
text to email plain text and html body.

We are worried that existing users might not understand where the
phabricator emails are coming from, so add a blurb that clearly
makes the connection to the old bugs.gnupg.org roundup tracker.

Test Plan:
Perform an action that triggers outgoing mail, then check if the
header is added correctly. Repeat once for HTML and once for plain
text.

• Signed-off-by: Marcus Brinkmann <marcus@gnupg.org>