Rework behaviour that a User is created if motions are forwarded

[Elblinator]

Currently:
If a motion is forwarded then a new User is created.
The surname of this user is the committee name -it was forwarded from- and the username is the committee name without spaces.
The User is a new participant in the meeting the motion was forwarded to and the user is assigned as a submitter in the forwarded motion.

To discuss behaviour:
Should the action to forward a motion maybe not create a new (inactive and no natural person) User who is then assigned as a submitter
But maybe the action to forward a motion should assign the Committee name as the extension for the submitter.
This would prevent a new automatically created (and very likely unused) account.
If the committee name is assigned in the extension-field then the name is still displayed everywhere the submitters are displayed.
This would also align the automatic forwarding behaviour with new feature described in OpenSlides/openslides-client#3429

[Elblinator]

This feature should be incorporated together with OpenSlides/openslides-client#3527

[luisa-beerboom]

This feature should be incorporated together with OpenSlides/openslides-client#3527

i.e. #2382

[luisa-beerboom]

So no more forwarding users from that point forward?
No chance the user/forwarding_committee_ids field will ever be filled again? That makes it kind of useless to have around.

Should we remove it?
Then what do we do with the old forwarding users? They still have that field. Do we keep them and their data around, just as if they'd been normal inactive users all the time?
Or do we migrate them away and write their names into the appropriate places in the meetings?

What do you think @rrenkert @r-peschke

[luisa-beerboom]

Talked with @rrenkert @Elblinator
The field will be removed, the users will not be removed. There will be a migration to remove the forwarding_committee_ids-relation that is all