Bug 26164: Replace OPAC table sort icons with SVG

Bug 26164: Replace OPAC table sort icons with SVG

This patch modifies the OAPC 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 OPAC 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 OPAC, for instance the table of
  checkouts under the logged-in user's "your summary" tab.
- Confirm that the sorting icons in the table headers look
  correct in each state: Sorted ascending, sorted descending, and
  unsorted.

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>