Bug 19030: Preserve link between order and subscription when editing an order

Bug 19030: Preserve link between order and subscription when editing an order

The aqorders.subscriptionid info is not sent to the template when the
order is edited. Which means we lose this link.

Test plan:
Create an order from a subscription
Edit the order
=> Without this patch, the aqorders.subscriptionid value is set to NULL
and items are created when receiving serial.

=> With this patch applied the link is preserved and expected behaviors
are preserved during all the acquisition workflow

You should also try and create several orders from the same subscription

Sponsored-by: BULAC - http://www.bulac.fr/
Signed-off-by: Séverine QUEUNE <severine.queune@bulac.fr>
Signed-off-by: Julian Maurice <julian.maurice@biblibre.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>