Append the request path when the site has it set

As a transitioneer
I would like to map whole legacy domains to subsections of my site 
So that I can avoid duplicated mapping work

Example http://www.ukspaceagency.bis.gov.uk/* became http://www.bis.gov.uk/ukspaceagency/* before GOV.UK existed. It would be useful to us to be able to redirect all traffic to the older domain to the path on the newer domain, so that we only need one mapping, not two.

The (Transition PR)[https://github.com/alphagov/transition/pull/272] will need to be deployed first because it has the schema changes.