Bug 8443: Suggestions publication year and copyright date

Bug 8443: Suggestions publication year and copyright date

Main patch correcting suggestions search :
- allow filtering on copyrightdate, manageddate and accepteddate
- entered dates remains in filter during search
- empty and null values are in the same tab
- null/empty values are not present in filers comboboxes

Signed-off-by: Chris Cormack <chris@bigballofwax.co.nz>
Signed-off-by: Katrin Fischer <Katrin.Fischer.83@web.de>
Signed-off-by: Jared Camins-Esakov <jcamins@cpbibliography.com>