R1:a15f07cc33d4

Allow Phabricator to be configured to use a public Reply-To address

Summary:
We already support this (and Facebook uses it) but it is difficult to configure
and you have to write a bunch of code. Instead, provide a simple flag.

See the documentation changes for details, but when this flag is enabled we send
one email with a reply-to like "D2+public+23hf91fh19fh@phabricator.example.com".
Anyone can reply to this, and we figure out who they are based on their "From"
address instead of a unique hash. This…