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 places.

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

Test Plan:
  - Used "Send Welcome Mail" from profile pages to send mail.
  - Hit "not admin", "disabled user", "bot/mailing list" errors.
  - Used `scripts/user/add_user.php` to send welcome mail.
  - Used "Create New User" to send welcome mail.
  - Verified mail with `bin/mail show-outbound`. (Cleaned up a couple of minor display issues here.)

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D19989