Bug 24877: Add link from vendor to linked subscriptions

Bug 24877: Add link from vendor to linked subscriptions

Koha shows the number of subscriptions linked to a vendor,
but there is no way to look them up. This adds a link to the
subscription search with the vendor name, so there is an easy way
to find the subscriptions.

To test:
- Be superlibrarian or have serials permission
- Make sure you have one or more subscriptions linked to vendors
- Go to the vendor's detail page in acquisitions
- Verify the number of subscriptions is now a link
- Verify the search results are correct when clicking on it
- Remove permissions (no serials, not superlibrarian)
- Verify now the number shows, but is not linked

Signed-off-by: Nazlı Çetin <nazli@devinim.com.tr>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>