Move DATETIME_FORMAT from app to app.utils

To avoid cyclical import issues

tests/app/celery/test_service_callback_tasks.py

@@ -5,8 +5,9 @@ import pytest
 import requests_mock
 from freezegun import freeze_time
 
-from app import (DATETIME_FORMAT, encryption)
+from app import encryption
 from app.celery.service_callback_tasks import send_delivery_status_to_service, send_complaint_to_service
+from app.utils import DATETIME_FORMAT
 from tests.app.db import (
     create_complaint,
     create_notification,

tests/app/celery/test_tasks.py

@@ -16,10 +16,7 @@ from notifications_utils.template import (
 )
 from notifications_utils.columns import Row
 
-from app import (
-    DATETIME_FORMAT,
-    encryption
-)
+from app import encryption
 from app.celery import provider_tasks
 from app.celery import tasks
 from app.celery.tasks import (
@@ -52,6 +49,7 @@ from app.models import (
     SMS_TYPE,
     ReturnedLetter,
     NOTIFICATION_CREATED)
+from app.utils import DATETIME_FORMAT
 
 from tests.app import load_example_csv