Bug 25846: Improve handling of multiple covers on catalog search results in the staff...

Bug 25846: Improve handling of multiple covers on catalog search results in the staff client

This patch modifies the template, JS, and CSS for the staff
interface catalog search results in order to gracefully handle multiple
cover images.

The changed version loops through any cover images which might be
embedded and checks that they are successfully loaded. Only
successfully-loaded images are shown. Only the first image is shown, and
the others can be "paged through" using generated navigation controls.

To test, apply the page and rebuild the staff client CSS
(https://wiki.koha-community.org/wiki/Working_with_SCSS_in_the_OPAC_and_staff_client).

Enable multiple cover image services. The patch was developed with these
services available:

 - Amazon
 - Local cover images (including multiple local cover images)
 - Coce (serving up Amazon, Google, and OpenLibrary images)
 - Images from the CustomCoverImages preference

Perform a variet of searches and confirm that cover images are
displaying correctly, whether there be 0, 1, 2, or more covers
available for each.

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>