Allow configuration of a task-creation email address

Summary: This lets you configure an email address which will create tasks when
emails are sent to it. It's pretty basic but should get us most of the way
there.
Test Plan: Configured an address and created a task via email. Replied to a task
via email to check that I didn't break that.
Reviewed By: tuomaspelkonen
Reviewers: davidreuss, jungejason, tuomaspelkonen, aran
CC: aran, epriestley, tuomaspelkonen
Differential Revision: 590

src/docs/configuration/configuring_inbound_email.diviner

@@ -2,7 +2,8 @@
 @group config
 
 This document contains instructions for configuring inbound email, so users
-may update Differential and Maniphest by replying to messages.
+may update Differential and Maniphest by replying to messages and create
+Maniphest tasks via email.
 
 = Preamble =
 
@@ -42,6 +43,11 @@ configured correctly, according to the instructions below -- will parse incoming
 email and allow users to interact with Maniphest tasks and Differential
 revisions over email.
 
+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
+has some mild security implications, see below.
+
 = Security =
 
 The email reply channel is "somewhat" authenticated. Each reply-to address is
@@ -74,6 +80,9 @@ practically, is a reasonable setting for many installs. The reply-to address
 will still contain a hash unique to the object it represents, so users who have
 not received an email about an object can not blindly interact with it.
 
+If you enable ##metamta.maniphest.public-create-email##, that address also uses
+the weaker "From" authentication mechanism.
+
 NOTE: Phabricator does not currently attempt to verify "From" addresses because
 this is technically complex, seems unreasonably difficult in the general case,
 and no installs have had a need for it yet. If you have a specific case where a