code: public static String getUrl(SlingHttpServletRequest request, String url, String extension) {
          return getUrl(request, url, null, extension);
      }
docs: Builds a (mapped) link to a path (resource path) without selectors and with the given extension.
  
  @param request   the request context for path mapping (the result is always mapped)
  @param url       the URL to use (complete) or the path to an addressed resource (without any extension)
  @param extension the extension (can be 'null'; should be 'html or '.html' by default)
  @return the mapped url for the referenced resource