Refactor high volume into serialised service model

Just looks a bit tidier and less repetitive.

I’ve only done this for the serialised service because:
- we’re only checking this in places where we’re already using the
  serialised service
- if we want to check this elsewhere there’s a good chance that new code
  should be using the serialised service, since it’ll itself be doing
  some kind of performance optimisation

app/v2/notifications/post_notifications.py

@@ -211,7 +211,7 @@ def process_sms_or_email_notification(
         template_with_content=template_with_content
     )
 
-    if service.id in current_app.config.get('HIGH_VOLUME_SERVICE') \
+    if service.high_volume \
         and api_user.key_type == KEY_TYPE_NORMAL \
             and notification_type in [EMAIL_TYPE, SMS_TYPE]:
         # Put service with high volumes of notifications onto a queue