NotificationType missing

[felixrindt]

Describe the bug
With the latest update we removed notification types, which lead to broken data after the update for some self hosters. We should consider testing for this scenario (and e.g. adding a Fallback Type) if we ever remove notification types or users uninstall a notification type.

To Reproduce
Steps to reproduce the behavior:

1. Change the type slug of a notification to some unknown slug
2. Try to access or display the notification in various web views

[jeriox]

fallback type introduced in #1718