Bug 14423: XSS issues in marc_subfields_structure

Bug 14423: XSS issues in marc_subfields_structure

1/ Hit a url like http://localhost:8081/cgi-bin/koha/admin/marc_subfields_structure.pl?op=add_form&tagfield=%22/%3E%3Cscript%3Ealert%28%27oh%20noes%27%29%3C/script%3E
2/ Notice all the alert boxes
3/ Apply patch
4/ Reload page, no more alerts
5/ Test functionality still works

Signed-off-by: Jonathan Druart <jonathan.druart@koha-community.org>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
(cherry picked from commit 91a8584aa845fb1695a46fe3b89197f7d1365d94)
Signed-off-by: Chris Cormack <chrisc@catalyst.net.nz>