web ui: Add explicit white border for QR code widget
    
    There was discussion in the ticket 9202 how to do that and the simplest
    way is to force a border via CSS. We have a CSS specifically for QR code
    widget, so use it.
    
    A default view is white, so the border will not be visible (white on
    white) but for dark backgrounds the border will be there clearly
    visible.
    
    Fixes: https://pagure.io/freeipa/issue/9202
    
    Signed-off-by: Alexander Bokovoy <abokovoy@redhat.com>
    Reviewed-By: Rob Crittenden <rcritten@redhat.com>