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/statistics.py

@@ -1,6 +1,9 @@
+from collections import defaultdict
 from datetime import datetime
 
 from app.models import NOTIFICATION_STATUS_TYPES, TEMPLATE_TYPES
+from app.utils import convert_utc_to_bst
+from app.dao.date_util import get_months_for_financial_year
 
 
 def format_statistics(statistics):
@@ -82,3 +85,24 @@ def _update_statuses_from_row(update_dict, row):
         update_dict['delivered'] += row.count
     elif row.status in ('failed', 'technical-failure', 'temporary-failure', 'permanent-failure'):
         update_dict['failed'] += row.count
+
+
+def create_empty_monthly_notification_status_stats_dict(year):
+    utc_month_starts = get_months_for_financial_year(year)
+    # nested dicts - data[month][template type][status] = count
+    return {
+        convert_utc_to_bst(start).strftime('%Y-%m'): {
+            template_type: defaultdict(int)
+            for template_type in TEMPLATE_TYPES
+        }
+        for start in utc_month_starts
+    }
+
+
+def add_monthly_notification_status_stats(data, stats):
+    for row in stats:
+        month = row.month.strftime('%Y-%m')
+
+        data[month][row.notification_type][row.notification_status] = row.count
+
+    return data