Bug 13425 - XSS in opac facets - Patch for master and 3.18

To Test
1/ Craft a url like /cgi-bin/koha/opac-search.pl?q=123&sort_by='"><script>prompt('Happy_Holidays')</script>&limit=123

It is important it must return results and facets

2/ Notice the js is executed
3/ Apply the patch test again

Signed-off-by: Mirko Tietgen <mirko@abunchofthings.net>
Popup is gone after applying the patch. Facet link still shows it but does not execute. It's gone after clicking the link.
Signed-off-by: Brendan Gallagher <brendan@bywatersolutions.com>
Signed-off-by: Chris Cormack <chrisc@catalyst.net.nz>