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 places.
The only functional change is that we no longer allow you to send welcome mail to disabled users.