make sure we combine stats that are already there

eg we have yesterdays data and want to top it up with todays deltas

'use a defaultdict to make this painless'

app/service/statistics.py

@@ -92,7 +92,7 @@ def create_empty_monthly_notification_status_stats_dict(year):
     # nested dicts - data[month][template type][status] = count
     return {
         convert_utc_to_bst(start).strftime('%Y-%m'): {
-            template_type: {}
+            template_type: defaultdict(int)
             for template_type in TEMPLATE_TYPES
         }
         for start in utc_month_starts
@@ -103,6 +103,6 @@ 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
+        data[month][row.notification_type][row.notification_status] += row.count
 
     return data