Bug 20339: Unify MARC21 ISBN/ISSN handling in XSL

Bug 20339: Unify MARC21 ISBN/ISSN handling in XSL

The code to show the ISBN and ISSN is repeated between the search results
and the detail view. Create a function to show the ISBN/ISSN, and call
that instead.

Test plan:
1) Apply patch
2) Search for any records, check that the ISBN and ISSN are shown
   correctly
3) Go to biblio detail pages, check that the ISBN and ISSN are shown
   correctly

Signed-off-by: Pasi Kallinen <pasi.kallinen@joensuu.fi>
Signed-off-by: claude <claude.brayer@cea.fr>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>