Bug 27850: Search link for 260 a and c in MARC21 XSLT OPAC display

Bug 27850: Search link for 260 a and c in MARC21 XSLT OPAC display

In MARC21 XSLT OPAC display, 260$b is a search link on 'Provider'.
$a and $c also have a search field, we can create search links.

Test plan:
1) Create 2 records with same 260 : two $a, one $b, two $c
2) Use XSLT display in OPAC
2) Look at first record details page in OPAC
3) Click on a $a link => you see both records
4) Click on a $c link => you see both records

https://bugs.koha-community.org/show_bug.cgi?id=27850

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>