Bug 24713: JavaScript error on staff client catalog search results page

Bug 24713: JavaScript error on staff client catalog search results page

This patch modifies the output of template toolkit variables so that
values in the in-page JavaScript are quoted. This avoids JavaScript
errors when the template variable is empty.

To test, apply the patch and perform a catalog search in the staff
client. Use the browser's developer tool to check for JavaScript errors
in the console. There should be none.

Test with AmazonCoverImages both on and off.

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>