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 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