Bug 20888: Allow use of boolean operator 'not' in item search

Bug 20888: Allow use of boolean operator 'not' in item search

Test plan:

 - Apply the patch
 - Check that you have a 'is not' option in the third box in item search
 - Check that you can exclude items from the search results with this 'is not' option
 - Check that you can still combine parameters with 'and' and 'or'
 - Check that the exclusion works for custom fields (see Administration > Item search fields)
 - Check that the exclusion works for custom fields linked to authorised values

Signed-off-by: Amandine Zocca <azocca@ville-montauban.fr>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>