Bug 9565: Deleting a biblio should alert/fail if there are existent subscriptions

Bug 9565: Deleting a biblio should alert/fail if there are existent subscriptions

Currently Koha will delete a biblio record if there are existent serial subscriptions.
This patch will prevent deleting the biblio until attached subscriptions are removed.

To test:
1) Add a subscription to a biblio, or check a biblio which has one attached already.
2) Remove all items if there are any.
3) Apply patch.
4) Confirm that the 'Delete record' item is disabled in the list.
5) Sign off.

Sponsored by: Lunds Universitetsbibliotek

Signed-off-by: Séverine QUEUNE <severine.queune@bulac.fr>
Signed-off-by: Frank Hansen <frank.hansen@ub.lu.se>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>