create new stats endpoint

the admin app currently calls get_detailed_service, which gets
notification stats, adds them on to the rest of the detailed service
endpoint, and returns them. However, the admin app then only looks at
the stats - it doesn't look at the rest of the service object.

This is called in a few high profile places - the dashboard, the
notification summary page, and when you send a job. By creating a
separate endpoint that ignores the rest of the service object (and no
marshmallow too!), the hope is that we'll improve some slowness we've
been seeing.

Note: The old detailed function will still need to stay - it's used
by get_services(detailed=True) for the platform admin page.

app/service/rest.py

@@ -170,6 +170,11 @@ def get_service_by_id(service_id):
     return jsonify(data=data)
 
 
+@service_blueprint.route('/<uuid:service_id>/statistics')
+def get_service_notification_statistics(service_id):
+    return jsonify(data=get_service_statistics(service_id, request.args.get('today_only') == 'True'))
+
+
 @service_blueprint.route('', methods=['POST'])
 def create_service():
     data = request.get_json()
@@ -411,12 +416,16 @@ def get_monthly_notification_stats(service_id):
 
 def get_detailed_service(service_id, today_only=False):
     service = dao_fetch_service_by_id(service_id)
+
+    service.statistics = get_service_statistics(service_id, today_only)
+    return detailed_service_schema.dump(service).data
+
+
+def get_service_statistics(service_id, today_only):
+    # today_only flag is used by the send page to work out if the service will exceed their daily usage by sending a job
     stats_fn = dao_fetch_todays_stats_for_service if today_only else dao_fetch_stats_for_service
     stats = stats_fn(service_id)
-
-    service.statistics = statistics.format_statistics(stats)
-
-    return detailed_service_schema.dump(service).data
+    return statistics.format_statistics(stats)
 
 
 def get_detailed_services(start_date, end_date, only_active=False, include_from_test_key=True):