Add whitenoise for static files

[alastair]

Finally get whitenoise in place for static files!

This will significantly improve our deployment process with docker, we'll now serve static files directly from django with all of the necessary caching headers, so we don't need any steps to move static files to nginx during deployment.

Deployment steps:
Need to remove the nginx location blocks for /static, we no longer need to copy files to appserver during deployment, and nginx doesn't need to copy files at startup.

It's possible that this will cause more load on django, and we may need to increase the number of web workers