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.

tests/app/celery/test_letters_pdf_tasks.py

@@ -93,13 +93,13 @@ def test_get_pdf_for_templated_letter_happy_path(mocker, sample_letter_notificat
     mock_get_letter_pdf_filename.assert_called_once_with(
         reference=sample_letter_notification.reference,
         crown=True,
-        sending_date=sample_letter_notification.created_at,
+        created_at=sample_letter_notification.created_at,
         ignore_folder=False,
         postage='second'
     )
 
 
-def test_get_pdf_for_templated_letter_non_existent_notification(notify_api, mocker, fake_uuid):
+def test_get_pdf_for_templated_letter_non_existent_notification(notify_db_session, mocker, fake_uuid):
     with pytest.raises(expected_exception=NoResultFound):
         get_pdf_for_templated_letter(fake_uuid)