Fix #932 Resend Activation sends invalid link

Moved the working code from the Users controller's register() method to
User_model (set_activation()) and called it from both register() and
resend_activation().

May need some additional work (like a new name and a better return
type). Currently returns an array containing a message (String) and a
Boolean indicating whether the email library returned an error. This was
done to avoid calling Template::set() directly from the model.
Additionally, the Settings controller should probably be sending the
activation emails as well when users are created from the admin site
area.