Bug 15140: Add MARC21 776 to OPAC and staff display

Bug 15140: Add MARC21 776 to OPAC and staff display

Adds code to opac and staff detail XSLT in order to show 776$t (or $a) and
a link based on 776$w (record control number). $w should be in the form:
    (MARCOrgCode)Record-number
For instance: (NL-AmRIJ)12345

Note: You should have enabled default opac/intranet XSLT view.

Test plan:
[1] Add two 776s (one with $t and $w and one with $t only)
[2] Verify that you see "Additional physical formats" on opac-detail. Does
    the link for $w work?
[3] Check intranet detail too.
[4] Empty 776$t and put text in $a. Check detail page again.

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Felix Hemme <felix.hemme@thulb.uni-jena.de>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>