Add reference for deleted accounts in supporters #3106
Description
context: OpenSlides/openslides-client#5324
[only the supporters here]
The technical wanted behaviour:
- The supporters need their own model
- It is not necessary that this model is sortable, currently it should not be supported to sort the supporters (they are automatically sorted alphabetically), if it is easier for the backend to allow sort then the backend can allow the sort -> the current behaviour should stay the same, the client should not support sort itself
- the supporters should not be deleted incadcadeDelete anymore
functional behaviour change:
If a motion has a supporter and this supporter account is deleted then the motion should "remember" that there once was a supporter for this motion