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 but just sets "reply-to"

to some address the install generates and still sends only one email

• private reply handler, provides a default generation mechanism or you can

override it and splits mail apart so we send one to each recipient

Test Plan:
Sent email from Maniphest and Differential with and without
reply-handler-domains set.

Reviewed By: aran
Reviewers: jungejason, tuomaspelkonen, aran
CC: aran, epriestley
Differential Revision: 254