move monthly notification stats from notification history to ft tables

it now uses the ft_notification_status table - nice and quick. if you
ask for the current tax year it'll top up the data with numbers from
the notification table.

the data is in exactly the same shape as the old endpoint - no changes
to the admin app are necessary

app/service/rest.py

@@ -12,11 +12,16 @@ from sqlalchemy.orm.exc import NoResultFound
 
 from app.dao import notifications_dao
 from app.dao.dao_utils import dao_rollback
+from app.dao.date_util import get_financial_year
 from app.dao.api_key_dao import (
     save_model_api_key,
     get_model_api_keys,
     get_unsigned_secret,
     expire_api_key)
+from app.dao.fact_notification_status_dao import (
+    fetch_notification_status_for_service_by_month,
+    fetch_notification_status_for_service_for_day
+)
 from app.dao.inbound_numbers_dao import dao_allocate_number_for_service
 from app.dao.organisation_dao import dao_get_organisation_by_service_id
 from app.dao.service_sms_sender_dao import (
@@ -33,7 +38,6 @@ from app.dao.services_dao import (
     dao_create_service,
     dao_fetch_all_services,
     dao_fetch_all_services_by_user,
-    dao_fetch_monthly_historical_stats_for_service,
     dao_fetch_monthly_historical_usage_by_template_for_service,
     dao_fetch_service_by_id,
     dao_fetch_stats_for_service,
@@ -89,7 +93,7 @@ from app.schemas import (
     notifications_filter_schema,
     detailed_service_schema
 )
-from app.utils import pagination_links
+from app.utils import pagination_links, convert_utc_to_bst
 
 service_blueprint = Blueprint('service', __name__)
 
@@ -397,15 +401,28 @@ def search_for_notification_by_to_field(service_id, search_term, statuses, notif
 
 @service_blueprint.route('/<uuid:service_id>/notifications/monthly', methods=['GET'])
 def get_monthly_notification_stats(service_id):
-    service = dao_fetch_service_by_id(service_id)
+    # check service_id validity
+    dao_fetch_service_by_id(service_id)
+
     try:
-        return jsonify(data=dao_fetch_monthly_historical_stats_for_service(
-            service.id,
-            int(request.args.get('year', 'NaN'))
-        ))
+        year = int(request.args.get('year', 'NaN'))
     except ValueError:
         raise InvalidRequest('Year must be a number', status_code=400)
 
+    start_date, end_date = get_financial_year(year)
+
+    data = statistics.create_empty_monthly_notification_status_stats_dict(year)
+
+    stats = fetch_notification_status_for_service_by_month(start_date, end_date, service_id)
+    statistics.add_monthly_notification_status_stats(data, stats)
+
+    now = datetime.utcnow()
+    if end_date > now:
+        todays_deltas = fetch_notification_status_for_service_for_day(convert_utc_to_bst(now), service_id=service_id)
+        statistics.add_monthly_notification_status_stats(data, todays_deltas)
+
+    return jsonify(data=data)
+
 
 def get_detailed_service(service_id, today_only=False):
     service = dao_fetch_service_by_id(service_id)