WIP: Integrating django-secure

For background info, please see the [django-developers thread](https://groups.google.com/d/topic/django-developers/t8ybImtdnpM/discussion).
We misread the check... it's checking the number of unique characters in the secret key. I've upped that to 10 (could probably be 20 though) and added a minimum length check of 50.