be less dependent on affected user

this would mainly be required for this feature request: https://github.com/nextcloud/deck/issues/7673

problem

activity events are always filtered by the affected user: https://github.com/nextcloud/activity/blob/master/lib/Data.php#L248

1. that forces apps to create an event for every user (for deck one event for each participant)
2. users without access at event creation can't read past history when gaining access(apps could recreate old events from the db, but that's very prone to have errors)

solutions (rather thoughts)

An option for apps to create events that are only related to an object and have "dynamic" affected users would be great, but that comes with quite some important implications (and probably some more I didn't think of):
What happens when users loose access, history lost as well?
Activity would need some way to get affected users from the app that created the event.
Data migration for events on existing instances...

Start with lib/Data.php around line 248 and read the linked Deck issue 7673 for the motivating use case. Before changing anything, determine how affected users are currently stored and filtered, then resolve the listed implications around access changes, app-provided users, and migration; the issue does not yet define a concrete completion state.