Fixed #17101 -- Integrated django-secure and added check --deploy option

For background info, please see the [django-developers thread](https://groups.google.com/d/topic/django-developers/t8ybImtdnpM/discussion).
Right -- but, I think it's better to be consistent with the other settings and make it opt-in in. Like the other settings, there'll be a warning if it's not enabled. If we enabled it by default, a user might not realize to turn it off if they do have an insecure domain.