rename letter get_folder_name args

`_now`? why would we ever use a different _now? instead say created_at,
because that's what it'll always be set to, even if we're replaying old
letters. We always set the folder name to when the letter was
created_at, or we might not know where to look to find it.

`dont_use_sending_date` doesn't really tell us what might happen if we
don't use it - the answer is we return an empty string. we ignore the
folder entirely. so lets call it that.

Also, remove use of freeze_gun in the tests, to prove that we don't use
the current time in any calculations. Also add an assert to a mock in
the get_pdf_for_templated_letter test, because we were mocking but not
asserting before, so the tests didn't fail when the function signature
changed.

app/celery/letters_pdf_tasks.py

@@ -60,12 +60,11 @@ def create_letters_pdf(self, notification_id):
 def get_pdf_for_templated_letter(self, notification_id):
     try:
         notification = get_notification_by_id(notification_id, _raise=True)
-
         letter_filename = get_letter_pdf_filename(
             reference=notification.reference,
             crown=notification.service.crown,
             sending_date=notification.created_at,
-            dont_use_sending_date=notification.key_type == KEY_TYPE_TEST,
+            ignore_folder=notification.key_type == KEY_TYPE_TEST,
             postage=notification.postage
         )
         letter_data = {
@@ -313,7 +312,7 @@ def process_sanitised_letter(self, sanitise_data):
             reference=notification.reference,
             crown=notification.service.crown,
             sending_date=notification.created_at,
-            dont_use_sending_date=True,
+            ignore_folder=True,
             postage=notification.postage
         )