Bug 34724: Remove inline onclick handler for opac-imageviewer.pl

Bug 34724: Remove inline onclick handler for opac-imageviewer.pl

This patch moves the click handler for thumbnails for opac-imageviewer.pl
out of the inline HTML and out to an existing <script> element.

Test plan:
0. Apply the patch (including dependent patches)
1. Go to
http://localhost:8081/cgi-bin/koha/admin/preferences.pl?tab=&op=search&searchfield=localcover
2. Change both *LocalCoverImages sysprefs to "Show"
3. Go to
http://localhost:8081/cgi-bin/koha/admin/preferences.pl?tab=&op=search&searchfield=AllowMultipleCovers
4. Change "AllowMultipleCovers" to "Allow"
5. Go to
http://localhost:8081/cgi-bin/koha/catalogue/detail.pl?biblionumber=29
6. Click "Images (0)" tab
7. Download or make two JPEG images
8. Upload the images
9. Go to http://localhost:8080/cgi-bin/koha/opac-imageviewer.pl?biblionumber=29
10. See one large image in the centre and two smaller thumbnails on the right side
11. Click on the thumbnail of the image that isn't displayed in the centre
12. Note that the image changes
13. Click the other thumbnail and note that the image changes back

PA amended patch: Added missing test plan step

Signed-off-by: Pedro Amorim <pedro.amorim@ptfs-europe.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>