Refactor the code that figures out what folder and filename to use for the letter pdf files.

Now we consistently use the created_at date, so we can always get the right file location and name.

The previous updates to this code were trying to solve the problem if a pdf being created at 17:29, but not ready to upload until 17:31 after the antivirus and validation check.
But in those cases we would have trouble finding the file.

app/service/send_notification.py

@@ -178,8 +178,9 @@ def send_pdf_letter_notification(service_id, post_data):
     )
 
     upload_filename = get_letter_pdf_filename(
-        notification.reference,
-        notification.service.crown,
+        reference=notification.reference,
+        crown=notification.service.crown,
+        sending_date=notification.created_at,
         is_scan_letter=False,
         postage=notification.postage
     )