Notifications are still on the old persistence system

(Bug, Investigate , Priority: Critical, Test Status: No automated tests yet , Reported By Justin du Coeur, )
Internals, Persistence, Notifications
Summary: This isn't actually breaking anything, but it's a very bad smell. And now that I'm adding userland ability to send notifications, it's going to become more problematic.
The job is conceptually simple: replace NotificationPersistence with an Akka-Persistence-based approach.
This doesn't have a hard deadline, but fixing it before A'19 would be a good idea, just from a scalability perspective. Probably the UserNotificationActor just needs to become a PersistentActor. The hard work is mostly going to be porting all the old data.