[ticket/12004] Support empty routes to app.php/ in path_helper

The symfony routing component allows us to use the path "/" for routes.
Therefore, we should be able to use example.com/app.php/ for controllers.
However, this currently does not properly work. The method get_web_root_path
incorrectly returns phpbb_root_path. Therefore, paths to images or files are
broken.

Ticket: http://tracker.phpbb.com/browse/PHPBB3-12004

PHPBB3-12004