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
handing.

Later we will need to add code to DifferentialMail.php to support
sending separate email to each email recipient to achieve better
security (see D226). The reply-to will be something like
D<revision_id>+<user_id>+<hash>@domain.com. We will create separate task
for it.

Test Plan:
tried comment on a revision from web UI and the email was
sent out as before without any change. When a subclass of
DifferentialReplyHandler is implemented and enabled, email's reply-to is
set and email text is added. Reply to the email with valid command did
create action to the revision.

Reviewed By: epriestley
Reviewers: tuomaspelkonen, epriestley, slawekbiel, dpepper
CC: aran, epriestley, jungejason
Differential Revision: 224