Prevent 404 page redirects flash

[adamhughes86]

The 404 page shows briefly before redirects - is it possible to get this script to halt page load/render until complete, so we don't get the quick 404 flash? I can't remember how feasible that is in fairly vanilla JavaScript/HTML

Originally posted by @yndajas in #1055 (review)

[adamhughes86]

This could be improved by amending the redirect script to hide the 404 container element until the script has completed