http_relative_path(+AbsPath,
-RelPath)Convert an absolute path (without host, fragment or search) into a path
relative to the current page, defined as the path component from the
current request (see http_current_request/1).
This call is intended to create reusable components returning relative
paths for easier support of reverse proxies.
If ---for whatever reason--- the conversion is not possible it simply
unifies RelPath to AbsPath.