Bug 26032: (follow-up) show 'is new' filter only if items.new_status is used

Bug 26032: (follow-up) show 'is new' filter only if items.new_status is used

Also adds a class 'item-new-status' to this filter to be able to hidde
even if items.new_status used.

Addition to test plan:
1) Apply on a database without Koha to MARC mapping on items.new_status
2) Check you dont see the 'is new' filter in items search form
3) Apply on a database with Koha to MARC mapping on items.new_status
4) Check you see class 'form-field item-new-status'

Signed-off-by: Séverine QUEUNE <severine.queune@bulac.fr>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>