Bug 13023: Nonpublic note for items should be mapped to a database column like itemnotes

Bug 13023: Nonpublic note for items should be mapped to a database column like itemnotes

An item's public notes are mapped to a db column, but the librarian only
notes are not. We need to map these so in the future we may display them
in places other than the editor.

Test Plan:
1) Create an item with a non-public note
2) Apply this patch
3) Run updatedatabase
4) Update your frameworks to map your nonpublic item note to items.itemnotes_nonpublic
5) Run misc/batchRebuildItemsTables.pl
6) Inspect the database, note that your non-public note is now in
   items.itemnotes_nonpublic

Signed-off-by: Paola Rossi <paola.rossi@cineca.it>
Signed-off-by: Brendan Gallagher <brendan@bywatersolutions.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@gmail.com>