Append the request path when the site has it set

This is useful where a site we are transitioning has moved to another site we
are transitioning, but on a sub-path, eg:
http://www.ukspaceagency.bis.gov.uk/*
became:
http://www.bis.gov.uk/ukspaceagency/*
before GOV.UK existed.

This feature means we can have the complex mappings in one place.