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

src/docs/configuration/configuring_inbound_email.diviner

@@ -43,6 +43,14 @@ configured correctly, according to the instructions below -- will parse incoming
 email and allow users to interact with Maniphest tasks and Differential
 revisions over email.
 
+If you don't want phabricator to take up an entire domain (or subdomain) you
+can configure a general prefix so you can use a single mailbox to receive mail
+on. To make use of this set ##metamta.single-reply-handler-prefix## to the
+prefix of your choice, and phabricator will prepend this to the 'Reply-To'
+mail address. This works because everything up to the first (optional) '+'
+character in an email-address is considered the receiver, and everything
+after is essentially "ignored".
+
 You can also set up a task creation email address, like ##bugs@example.com##,
 which will create a Maniphest task out of any email which is set to it. To do
 this, set ##metamta.maniphest.public-create-email## in your configuration. This