Bug 24157: New permission - merge_invoices

Bug 24157: New permission - merge_invoices

Add a new permission to merge invoices

Test plan:
- Remove the new permission "merge_invoices" for a given patron,
use it to log in into Koha
- Create 2 invoices, try to merge them
=> There is no way to merge it
- Add the permission
=> Now you can merge the invoices

Sponsored-by: Galway-Mayo Institute of Technology
Signed-off-by: Alex Arnaud <alex.arnaud@biblibre.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>