Notifications: system notifications should also go to assigned users

[dasgarner]

In #2957 we fixed a problem whereby system notifications were not being sent to the mail_to if one had been specified. In fixing this problem the logic as it was stated now works, which is to say that when a notification is a system notification it does not send to the assigned users.

In hindsight this logic seems wrong - a notification should go to all assigned users, and additionally the mail_to user if it is a system notification.