[ticket/11997] Fix redirects from inside controllers

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
[ticket/11997] Fix missing global

PHPBB3-11997
[ticket/11997] Add functional test for redirects in controller

PHPBB3-11997