Add a page to show delivery rates week-by-week

Implements
https://github.com/alphagov/notifications-api/pull/286

Will always show weeks as Monday to Sunday.

app/notify_client/statistics_api_client.py

@@ -20,3 +20,14 @@ class StatisticsApiClient(BaseAPIClient):
             url='/service/{}/notifications-statistics'.format(service_id),
             params=params
         )
+
+    def get_7_day_aggregate_for_service(self, service_id, date_from=None, week_count=None):
+        params = {}
+        if date_from is not None:
+            params['date_from'] = date_from
+        if week_count is not None:
+            params['week_count'] = week_count
+        return self.get(
+            url='/service/{}/notifications-statistics/seven_day_aggregate'.format(service_id),
+            params=params
+        )