LibWeb: Fix bug where viewing resource schemed directory urls crashes

[bplaat]

This pr removes the @resource_directory_url@ template variable and uses the new resource:// scheme to get to the right icons.

It also adds some code that when viewing a resource schemed directory url a generated directory page is responded instead of the browser crashing on a VERIFY_NOT_REACH() in Resource.cpp.

I also get the path to the directory.html and error.html page with resources to reduce code.

[bplaat]

Thanks for the review Flynn, I've fixed your remarks