refactor(DocumentReceiverElement): make AddedWhen nullable and add ChangedWho-property

EnvelopeGenerator.Application/EnvelopeReceivers/Queries/ReceiverAlreadySignedQuery.cs

@@ -71,7 +71,7 @@ public class ReceiverAlreadySignedQueryHandler : IRequestHandler<ReceiverAlready
         return await _repo.Read()
             .Where(er => er.Envelope!.Uuid == request.Envelope.Uuid)
             .Where(er => er.Receiver!.Signature == request.Receiver.Signature)
-            .Where(er => er.Envelope!.History.Any(hist => hist.Status == EnvelopeStatus.DocumentSigned))
+            .Where(er => er.Envelope!.Histories.Any(hist => hist.Status == EnvelopeStatus.DocumentSigned))
             .AnyAsync(cancel);
     }
 }