Backend should generate random orga id, not frontend

[fm3]

Quoting @MichaelBuessemeyer in #8193 (comment):

To me it is also kinda strange, why the frontend generates this id string and not the backend 🤔. Maybe this could be changed in the future?

I think this has historical reasons only and could indeed be moved to the backend. Not sure I know all relevant cases, though.

We could also discuss if a random string is the best choice. Since this is already not a mongoId and some old organizations have human-readable (albeit sanitized) ids, maybe a new format would be a good idea. We could also concat a random part and a human-readable part to prevent collisions?