router: implement RelativeURLPath

We can use this for generating correct relative URLs in pages that
we serve (we'll use it initially in the generated SVGs for bundles).

Also add some copyright notices that were needed.
I don't think that would work as we're building up a slice of strings (path elements) here, not a single string, and if we added a single element with all those .. elements in, we would add an extra / when appending targOnly.