Ref T5791. This diff adds a "sensitive" flag to PhabricatorMetaMTAMail, defaults it to true in the constructor, and then sets it to false in teh application transaction editor. Assumption here is that sensitive emails are basically all the emails that don't flow through the application transaction editor.
This diff also gets a basic "mail view" page up and going.
This diff also fixes a bug writing recipient edges; the actor was being included.
This bug also fixes a querying bug; we shouldn't do the automagic join of $viewer is recipient or $viewer is actor if folks are querying for recipients or actors already. The bug manifested itself as having the "inbox" be inbox + outbox.