Allow for filtering the notifications

[christianlupus]

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Feature description

The current notification system piles up quite some information that cannot be removed (#5585). An alternative to (permanently) removal of the notifications would be to allow to classify and filter them accordingly.

I can think of thinks like

• system messages
• calendar events
• birthdays
• app-specific events (separated by app)
• file changes/updates/shares/...
• core messages
• found updates for apps/core
• connection issues
• conflicting files (possible [Bug]: Conflict files not listed. #4954)
• ...

It would maybe make the removal of notifications less critical if one could toggle a filter which events the user want to see and which should be hidden. That would allow also a sensible default per user (some might be tech-savvy while other want the bare minimum or distraction).

[jospoortvliet]

So first, users can configure notifications on /settings/user/notifications
and second, there's been some work done to make it easier to clean up notifications.

I personally still have doubles and am not crazy about how it works, agreed, but (more) client side configuration to me feels like the wrong solution...