Implement a new type of "Moderator" notification type for forums
This notification will be triggered also by unapproved messages. The UI will show the subscription link only to forum moderators, using a different color (customizable via css, vanilla color is red). Message content will always be hidden for such notifications, so user will need to have access to new message.
Step in the direction of implementing a new kind of "moderator notification": move current notification behavior in a private api, remove dependency on message approval in message::new (also avoids a query)