Fix username var in params

[xgaia]

Hello @wheelybird !

I updated the new_user.php file to get the username params when an account is added with user in the params url.

I also replace deprecated FILTER_SANITIZE_STRING with FILTER_SANITIZE_FULL_SPECIAL_CHARS.

Regards,

Xavier

[wheelybird]

Thanks. I'll need to take a look at that as I think there's a reason that the generate_username function is called there.