Weird behavour if user who voted is deleted

[Elblinator]

Reproduction:

1. Create a participant, add them to delegate group
2. Start a poll (in my test it was non-nominal)
3. vote with the new participant
4. stop/publish the poll
5. Delete the user
6. tadaaa, errors from AU:

autoupdate-1  | 12:40:14 ERROR  Error: creating key list from notification: the key/fqfield is invalid: history_entry/52/model_id_user_id
autoupdate-1  | creating key list from notification: the key/fqfield is invalid: history_entry/57/model_id_user_id

Additional:
Same behaviour if the user delegated to another user and they voted for them
Also same behaviour if the deleted user only voted for someone else

[ostcar]

The problem is this field: https://github.com/OpenSlides/openslides-meta/blob/e28d5acde420f6ddb4beeb9504410a46362287c8/collections/history_entry.yml#L15

It is from a histroy entry and directy to the user.

This needs a solution in the datastructure. Options are:

• The backend has to delete all history entries, when it deletes a user
• Do not delete uses with history entries, but rename them to "deleted"
• Make the field history_entry/model_id nullable

Also: It seems, that the notification sends the key history_entry/52/model_id_user_id. This is not a valid key, but the name of a helper field in postgres. This has to be fixed.

I move this issue to meta, since there is nothing to do in the autoupdate