Bug 9968: Fix wrong index for Standard number in advanced and label search

Bug 9968: Fix wrong index for Standard number in advanced and label search

'Standard number' indicates that you can search ISBN, ISSN and other
standard numbers, but we are using the wrong index and only get a
very limited search for biblionumber instead.

This patch changes the index from sn to ident.

To test:
- Check both staff and OPAC advanced search:
  - Search for Standard number = ISBN
  - Search for Standard number = ISSN
  Both should work.
  Toggle [Intranet|OPAC]NumbersPreferPhrase system preferences
- Check the label creator
  - Add a new batch
  - Use 'add items' option and search for Standard number

Signed-off-by: Michal Denar <black23@gmail.com>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>