Many system notifications might not be displayed

System Notifications (The red ones on the System/Overview page)
are only published once per type.
If there is one notification for e.g. an Archiving error, the second one will be silently dropped.

This is probably fine and works as designed.

However, there are currently roughly 15 different subsystems all sharing the `GENERIC` type:

https://github.com/Graylog2/graylog2-server/blob/master/graylog2-server/src/main/java/org/graylog2/notifications/Notification.java#L67

That would mean for instance that if there is an error in the reporting plugin, archiving errors won't be shown.

## Possible Solution

- Introduce more types, or use class names for subtypes
- Publish multiple notifications per type? (Maybe limited, because this might overload the UI)