Switch to django.contrib.staticfiles

This allows us to pull in new features and bugfixes from the mainline of development
for both django.contrib.staticfiles and django-pipeline, rather than using our
forks of these projects.

This PR was inspired by #1889 -- which would not have broken the build if we were using `django.contrib.staticfiles`. The bug that bit us [was caught and fixed by the Django development team a year ago](https://github.com/django/django/commit/1aa0d8ac4d3149fad6ebe80834d9e6f83021cf55).