Pass letter logo filename to template preview

We now pass through `filename` to the Template Preview `/preview.<filetype>'
endpoint in addition to the `dvla_org_id`. Template Preview will be
changed to only use `filename`, and once this has been done, we can change
the code to only pass through `dvla_org_id` instead of passing through
both pieces of information.

tests/app/test_template_previews.py

@@ -49,7 +49,9 @@ def test_from_database_object_makes_request(
 ):
     resp = Mock(content='a', status_code='b', headers={'c': 'd'})
     request_mock = mocker.patch('app.template_previews.requests.post', return_value=resp)
-    mocker.patch('app.template_previews.current_service', dvla_organisation='123')
+    mocker.patch('app.template_previews.current_service',
+                 dvla_organisation='123',
+                 letter_logo_filename='hm-government')
     template = mock_get_service_letter_template('123', '456')['data']
 
     ret = partial_call(template=template)
@@ -62,6 +64,7 @@ def test_from_database_object_makes_request(
         'letter_contact_block': None,
         'template': template,
         'values': None,
+        'filename': 'hm-government',
         'dvla_org_id': '123',
     }
     headers = {'Authorization': 'Token my-secret-key'}