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.
PHPBB3-12004
The redirect url currently uses the web root path. However as we prepend the
full board url later, we need to remove the relative web root path and prepend
the normal root path again. Otherwise redirects from inside routes will not
work as intended.
PHPBB3-11997
