[develop] Get $root_path via a function

Built the $root_path from the URL path and the existing $phpbb_root_path
via a function in /includes/functions.php

This is my first commit to phpBB so I apoligize in advance if I did
something wrong

PHPBB3-11486
Built the $root_path from the URL path and the existing $phpbb_root_path

Changed to use the new get root function

PHPBB3-11486
Built the $root_path from the URL path and the existing $phpbb_root_path

Fixed the two methods to use the new get_root_path() method

PHPBB3-11486