R1:5537e29ee8e9

Move "Welcome" mail generation out of PhabricatorUser

Summary:
Ref PHI1027. Currently, `PhabricatorUser` has a couple of mail-related methods which shouldn't really be there in the long term. Immediately, I want to make some adjusments to the welcome email.

Move "Welcome" mail generation to a separate class and consolidate all the error handling. (Eventually, "invite" and "verify address" email should move to similar subclasses, too.) Previously, a bunch of errors/conditions got checked in multiple…