Only aggregate status when necessary for a service

This takes a similar approach to the nightly deletion task so that
we only create sub-tasks when there are actually notifications to
aggregate for a given type and day [1].

We're making this change to stop the duplication errors we're getting
at the moment and ensure the task can scale to more messages and more
services. There are two parts to this:

- Each subtask should now run within the 5 minute visibility timeout.
However, they may still be duplicated if the parent task overruns [2].

- The parent task creates a mininal number of subtasks, and the query
to determine this is very fast for a normal process day (milliseconds).

Since all tasks will run quickly, there should be no more duplication.

In order to test this more nuanced task, I rewrote the tests:

- One test checks the subtask is called correctly.
- One test checks we create all the right subtasks.

[1]: https://github.com/alphagov/notifications-api/pull/3381
[2]: https://docs.google.com/document/d/1MaP6Nyy3nJKkuh_4lP1wuDm19X8LZITOLRd9n3Ax-xg/edit#heading=h.q3intzwqhfzl

app/celery/reporting_tasks.py

@@ -3,7 +3,7 @@ from datetime import datetime, timedelta
 from flask import current_app
 from notifications_utils.timezones import convert_utc_to_bst
 
-from app import db, notify_celery
+from app import notify_celery
 from app.config import QueueNames
 from app.cronitor import cronitor
 from app.dao.fact_billing_dao import (
@@ -14,7 +14,8 @@ from app.dao.fact_notification_status_dao import (
     fetch_status_data_for_service_and_day,
     update_fact_notification_status,
 )
-from app.models import EMAIL_TYPE, LETTER_TYPE, SMS_TYPE, Service
+from app.dao.notifications_dao import get_service_ids_with_notifications_on_date
+from app.models import EMAIL_TYPE, LETTER_TYPE, SMS_TYPE
 
 
 @notify_celery.task(name="create-nightly-billing")
@@ -97,8 +98,11 @@ def create_nightly_notification_status():
         for i in range(days):
             process_day = yesterday - timedelta(days=i)
 
-            for (service_id,) in db.session.query(Service.id):
+            relevant_service_ids = get_service_ids_with_notifications_on_date(
+                notification_type, process_day
+            )
 
+            for service_id in relevant_service_ids:
                 create_nightly_notification_status_for_service_and_day.apply_async(
                     kwargs={
                         'process_day': process_day.isoformat(),