Seems like a package parameter makes sense since the semantics is specific to each package.
It seems wrong to allow uses to not send notifications, if someone has subscribed. What makes more sense is to create a specific notification type for each type of notification.
For example. For forums you can subscribe to one thread or an entire forum.
For bugtracker you can subscribe to tickets you created, all tickets you are involved in, or all tickets.
File storage just needs to provide the correct options.
Not sure what to do with xowiki. As its a wiki, its hard to tell. Perhaps what people would want is a batched notification for that instead of instant.
