Thoughts on notifications.

In the database, Notifications are associated with an Activity, such as Follow, Boost, Mention, etc. Mastodon and Pleroma both do it this way - no Activity, no Notification.

I think there are times we want to create a Notification without an Activity, such as notifying site moderators when a report is received, notifying donors when their payment was declined, notifications about group posts, etc. These events are not necessarily Activities, because usually an Activity is an event that can federate. I believe that Notifications should be optionally associated with a custom model instead of an Activity; possibly even with an App.