Refs #5127 revert previous commit as this was not the fix
Thanks for sending me access to the server so I could check myself
Fixes #5127 Set Base URL used in the AJAX request to `?` instead of `index.php?`

 It is set to '?' rather than 'index.php?' to increase chances that it works
 including for users who have an automatic 301 redirection from index.php? to ?
 POST values are missing when there is such 301 redirection. So by by-passing
 this 301 redirection, we avoid this issue.