WIP: Integrating django-secure

For background info, please see the [django-developers thread](https://groups.google.com/d/topic/django-developers/t8ybImtdnpM/discussion).
I think we just didn't want to make the check overly aggressive, but I wouldn't have a problem bumping this up, even to 50. We could ask @dstufft or @PaulMcMillan how many characters they consider a `SECRET_KEY` ought to have. Chances are almost everyone is using a `SECRET_KEY` generated by `startproject`, so probably any value of 50 or less is fine.