WIP: Integrating django-secure

For background info, please see the [django-developers thread](https://groups.google.com/d/topic/django-developers/t8ybImtdnpM/discussion).
`all other possible measures` is wording that is vulnerable to pedantry (taking down the site entirely is a "possible measure"!). I would change to "and you should ensure that you are also validating and sanitizing all input to prevent XSS attacks."