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).
If a site serves user-uploaded files, and following typical Django deployment practice, then those files would not be served by Django but directly from the media directory by nginx/apache/etc. So in that case, setting this header in Django does not help at all. For that, one would have to set the headers in nginx/apache.

That may be a good approach for many of these headers anyways. We do mention a recommendations like this for SSL redirects, but then for performance reasons.