rename sending_date to created_at

we don't name letters based on the day we send them on, rather, the day
we create them on. If we process a letter for a second time for whatever
reason, even if it's a couple of days later, it'll still go in a folder
based on the created_at timestamp. There's still a slight confusion,
however - if the timestamp is after 5:30pm, the folder will be for the
day after. However, still the day after creation, so I think created_at
still makes the most sense.

Remove the term `sending_date` to try and make this relationship more
apparent.

app/service/send_notification.py

@@ -195,7 +195,7 @@ def send_pdf_letter_notification(service_id, post_data):
     upload_filename = get_letter_pdf_filename(
         reference=notification.reference,
         crown=notification.service.crown,
-        sending_date=notification.created_at,
+        created_at=notification.created_at,
         ignore_folder=False,
         postage=notification.postage
     )