When password auth is not enabled, don't tell users to set a password in welcome email

Summary:
See PHI1027. Currently, the "Welcome" mail always tells users to set a password. This definitely isn't helpful if an install doesn't have password auth enabled.

We can't necessarily guess what they're supposed to do, so just give them generic instructions ("set up your account"). Upcoming changes will give administrators more control over the mail content.

Test Plan: Sent both versions of the mail, used bin/mail show-outbound to inspect them for correctness.

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D19990