make persist_notification require kwargs

when functions get as big as that, it's confusing to try and work out what
things are what. By including a * as the first arg, we require that anyone
calling the function has to use kwargs to reference the parameters

app/notifications/process_notifications.py

@@ -35,6 +35,7 @@ def check_placeholders(template_object):
 
 
 def persist_notification(
+    *,
     template_id,
     template_version,
     recipient,