R1:71efb46ba77d

Support email multiplexing for private Reply-To addresses

Summary:
Provide a base PhabricatorMailReplyHandler class which handles the plumbing for
multiplexing email if necessary and supporting public and private reply handler
addressses. DifferentialReplyHandler now extends it, and a new
ManiphestReplyHandler also does.

The general approach here is that we have three supported cases:

- no reply handler, default config, same as what we're doing now
- public reply handler, requires overriding classes…