[ticket/11997] Do not check if file or dir we redirect to exist

The redirect function will now properly redirect to where we want it to.
It will no longer try to check if the file or directory we redirect to exist.
This will ensure compatibility with the new routes.

PHPBB3-11997
[ticket/11997] Add remove_web_root_path() in order to prevent incorrect URLs

Adding a call to this method to the redirect function will make sure that we
do not end up with incorrect URLs after using append_sid().

PHPBB3-11997
[ticket/11997] Use path_helper in in foo/bar extension for redirect URLs

By using path_helper's clean_url() method, we'll be able to properly check
the full URL instead of just parts of the expected URL.

PHPBB3-11997