Enable support for a single reply-handler for outbound emails

Summary:
This allows you to configure a single mailbox for all mail sent by phabricator,
so you
can keep a mailaddress like bugs@example.com and don't need a catchall on your
domain/subdomain.

Test Plan:
Enabled and disabled suffix. Saw mails generated have to correct prefix. Also
piped raw mails
into the scripts/mail/mail_handler.php and ensured comments went into
phabricator for both maniphest
and differential.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, epriestley

Differential Revision: 815