new users should get a welcome email

[timdiggins]

when a user is added through the Users controller, a welcome email should be sent (using delayed job).

The welcome email should have a link to the password resets page, so they can set their password (like a "forgot password" link).

(as a future enhancement, make the password reset page invite them to "Set a password" rather than "Reset their password", if they haven't logged in yet)