Add a registration request for the user.

        :rtype : RegisterUser

def add_registration(self, username, first_name, last_name, email, password=""):
        """
            Add a registration request for the user.

        :rtype : RegisterUser
        """
        register_user = self.appbuilder.sm.add_register_user(
            username, first_name, last_name, email, password
        )
        if register_user:
            if self.send_email(register_user):
                flash(as_unicode(self.message), "info")
                return register_user
            else:
                flash(as_unicode(self.error_message), "danger")
                self.appbuilder.sm.del_register_user(register_user)
                return None