Bug 34453: Update background of quick spine label pop-up

Bug 34453: Update background of quick spine label pop-up

This patch updates the quick spine label popup so that the footer uses
the same markup as the recently updated Z39.50 popup window in
cataloging.

To test, apply the patch and rebuild the staff interface CSS.

Go to Cataloging -> Quick spine label creator.

 - Test the form with both an existing barcode and a non-existent
   barcode.
   - In the pop-up window, confirm that the footer looks correct and
     that the buttons in the footer, in both cases, work correctly.

Signed-off-by: Sam Lau <samalau@gmail.com>
Signed-off-by: Caroline Cyr La Rose <caroline.cyr-la-rose@inlibro.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>