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.

tests/app/celery/test_letters_pdf_tasks.py

@@ -62,7 +62,10 @@ def test_get_pdf_for_templated_letter_happy_path(mocker, sample_letter_notificat
         letter_branding = create_letter_branding(name=branding_name, filename=logo_filename)
         sample_letter_notification.service.letter_branding = letter_branding
     mock_celery = mocker.patch('app.celery.letters_pdf_tasks.notify_celery.send_task')
-    mocker.patch('app.celery.letters_pdf_tasks.get_letter_pdf_filename', return_value='LETTER.PDF')
+    mock_get_letter_pdf_filename = mocker.patch(
+        'app.celery.letters_pdf_tasks.get_letter_pdf_filename',
+        return_value='LETTER.PDF'
+    )
     get_pdf_for_templated_letter(sample_letter_notification.id)
 
     letter_data = {
@@ -87,6 +90,14 @@ def test_get_pdf_for_templated_letter_happy_path(mocker, sample_letter_notificat
         queue=QueueNames.SANITISE_LETTERS
     )
 
+    mock_get_letter_pdf_filename.assert_called_once_with(
+        reference=sample_letter_notification.reference,
+        crown=True,
+        sending_date=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):
     with pytest.raises(expected_exception=NoResultFound):

tests/app/letters/test_letter_utils.py

@@ -1,6 +1,7 @@
 import pytest
 from datetime import datetime
 
+import dateutil
 import boto3
 from flask import current_app
 from freezegun import freeze_time
@@ -161,7 +162,7 @@ def test_get_letter_pdf_filename_returns_correct_filename_for_test_letters(
         notify_api, mocker):
     sending_date = datetime(2017, 12, 4, 17, 29)
     filename = get_letter_pdf_filename(reference='foo', crown='C',
-                                       sending_date=sending_date, dont_use_sending_date=True)
+                                       sending_date=sending_date, ignore_folder=True)
 
     assert filename == 'NOTIFY.FOO.D.2.C.C.20171204172900.PDF'
 
@@ -216,7 +217,7 @@ def test_upload_letter_pdf_to_correct_bucket(
         reference=sample_letter_notification.reference,
         crown=sample_letter_notification.service.crown,
         sending_date=sample_letter_notification.created_at,
-        dont_use_sending_date=is_precompiled_letter
+        ignore_folder=is_precompiled_letter
     )
 
     upload_letter_pdf(sample_letter_notification, b'\x00\x01', precompiled=is_precompiled_letter)
@@ -243,7 +244,7 @@ def test_upload_letter_pdf_uses_postage_from_notification(
         reference=letter_notification.reference,
         crown=letter_notification.service.crown,
         sending_date=letter_notification.created_at,
-        dont_use_sending_date=False,
+        ignore_folder=False,
         postage=letter_notification.postage
     )
 
@@ -293,7 +294,7 @@ def test_move_failed_pdf_scan_failed(notify_api):
     assert filename not in [o.key for o in bucket.objects.all()]
 
 
-@pytest.mark.parametrize("freeze_date, expected_folder_name",
+@pytest.mark.parametrize("timestamp, expected_folder_name",
                          [("2018-04-01 17:50:00", "2018-04-02/"),
                           ("2018-07-02 16:29:00", "2018-07-02/"),
                           ("2018-07-02 16:30:00", "2018-07-02/"),
@@ -309,15 +310,14 @@ def test_move_failed_pdf_scan_failed(notify_api):
                           ("2018-01-02 23:30:00", "2018-01-03/"),
                           ("2018-01-03 00:30:00", "2018-01-03/"),
                           ])
-def test_get_folder_name_in_british_summer_time(notify_api, freeze_date, expected_folder_name):
-    with freeze_time(freeze_date):
-        now = datetime.utcnow()
-        folder_name = get_folder_name(_now=now, dont_use_sending_date=False)
+def test_get_folder_name_in_british_summer_time(notify_api, timestamp, expected_folder_name):
+    timestamp = dateutil.parser.parse(timestamp)
+    folder_name = get_folder_name(created_at=timestamp, ignore_folder=False)
     assert folder_name == expected_folder_name
 
 
 def test_get_folder_name_returns_empty_string_for_test_letter():
-    assert '' == get_folder_name(datetime.utcnow(), dont_use_sending_date=True)
+    assert '' == get_folder_name(datetime.utcnow(), ignore_folder=True)
 
 
 @mock_s3