Add notifications support

So far this is set up in the following way:

This is not a notification that users can manage in their UCP. This comes direct from an Admin, so users should not be able to ignore them, IMO.

This does not send out email notification. This should be (and I hope I set it up this way) an in-browser-phpBB notification only.

UI - The Board Rules settings page has a Notify Users button that the Admin can hit to send this notification out to all registered users (except for BOTS and GUESTS of course). Rule anchors are supported in the notification link (but not implemented yet).