R1:162f34b8c8e7

Add reply handler for differential revision

Summary:
add email reply handler so that the user can reply to a
differential email to act on the revision. It generates the reply-to
email address, creates email body text with supported commands list, and
handle the action request on the differential revision.

Right now the reply-to handing is disabled in the config file. But a
site using Phabricator can enable it and implement a class
inheriting from DifferentialReplyHandler to enable customized email…