freeipa_webui: handle scrolling for add_user button
    
    In https://openqa.fedoraproject.org/tests/1051360 Firefox showed
    an "Open previous tabs?" bar which put this button just off the
    bottom of the screen, so we need to scroll to it if that happens.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>