Permanently redirect (301) Whitehall assets from gov.uk to the asset host

[chrisroos]

alphagov/whitehall#3627 redirects Whitehall assets from www.gov.uk to the asset host (e.g. assets.publishing.service.gov.uk). We've added it as a temporary (302) redirect but it should really be a permanent (301) redirect as we don't want any assets being served by www.gov.uk in future.

See issue #348 for more information about the redirection we've put in place.