Restarting (and likely loading) screen is missing assets if prefixed infront of proxy

When restarting a jenkins server (<jenkinsurl>/restart), the static assets (such as css/js) are not prefixed, so if jenkins is sitting behind a proxy server, the assets are not coming from the right place.

This appears to work fine in standalone mode (java -jar war), but I'm not sure why.

Looks like the jelly files reference resURL, but resURL is undefined for the restarting template.