Bug 26060: Replace staff interface table sort icons with SVG

Bug 26060: Replace staff interface table sort icons with SVG

This patch modifies the staff interface CSS to convert table sorting
icons from PNG to SVG. These icons are added via background-image
data-urls, so only the SCSS file is modified.

To test, apply the patch and rebuild the staff interface CSS
(https://wiki.koha-community.org/wiki/Working_with_SCSS_in_the_OPAC_and_staff_interface).

 - Clear your browser cache if necessary
 - View any DataTable in the staff interface, for instance when checking
   out to a patron with existing checkouts, the catalog detail page for
   a title with holdings, the table of saved reports.
 - Confirm that the sorting icons in the table headers look correct in
   each state: Sorted ascending, sorted descending, and unsorted.

Signed-off-by: Sally <sally.healey@cheshiresharedservices.gov.uk>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>