Add reference for deleted accounts in submitters, spokesperson and editors #3105
Description
context: OpenSlides/openslides-client#5324
[missing in this issue "supporter" bc they need will be dealt with in another issue]
The technical wanted behaviour (if understood correctly):
the three instances should not be deleted incadcadeDelete anymore
functional behaviour change:
If a motion has a submitter (or the other two) and this submitter account is deleted then the motion should "remember" that there once was a submitter for this motion