Remove collectstatic-disabled, and use PyPi modules

- Use PyPi versions of django-versioned-static, django-static-root-finder & django-asset-server-url in requirements
- Remove django-collectstatic-disabled

We decided we don't need collectstatic-disabled because
actually in most cases, anything using our django project
should be informed directly that collectstatic isn't available.
They should handle the error gracefully themselves.

If we find it is breaking something, not having collectstatic available,
then we will re-introduce the dependency