Bug 17107 - Add ident and Identifier-standard to known indexes

Bug 17107 - Add ident and Identifier-standard to known indexes

Add ident and Identifier-standard to known indexes in C4::Search::getIndexes().
Those indexes can be very useful, for example for IdRef feature.

Test plan :
- Make sure some records have a field indexed with Identifier-standard, ISBN=1234 for example
- Perform a search /cgi-bin/koha/opac-search.pl?idx=ident,phr&q=1234
=> you find the record
- Perform a search /cgi-bin/koha/opac-search.pl?q=ident:1234
=> Without patch : you get no results
=> With patch    : you find the record
Idem for 'Identifier-standard'

Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>