[ticket/12261] Remove web root path from login redirect url

The redirect url must be relative to the ucp.php and not relative to the
current page.

PHPBB3-12261
[ticket/12261] Add a functional test for redirect after using login_box()

PHPBB3-12261
[ticket/12261] Validate the redirect hidden field before submitting the form

PHPBB3-12261
[ticket/12261] Fix doc block

PHPBB3-12261
[ticket/12261] Mark test as incomplete, something is wrong with sessions

PHPBB3-12261
Merge pull request #2104 from nickvergessen/ticket/12261

[ticket/12261] Remove web root path from login redirect url