Send e-mails with a Sender: header for better DKIM support.
Some spam filters get caught up on Review Board's From: forging. The address
doesn't really match the source address, causing them to expect spam or
some other malicious e-mail.
We now use the Sender header (and the X-Sender variant) in the e-mails.
Servers can use this to find the true sender of the e-mail, instead of assuming
it's the value in From. By default, this is noreply@<domain>, but it can be
changed in the admin UI.
This should improve our DKIM support.
The tradeoff is that some e-mail clients (Outlook, for instance) will show
some annoying "on behalf of" text where the sender's e-mail address is
typically shown. We may want to add the ability to disable this on installs.
While annoying, this is the behavior of mailman as well. See
Unit tests pass.
The resulting debug output looked fine, but I couldn't test actual e-mails
from this connection, because outgoing port 25 is blocked.