Bug 7228: can't add items in an order with Modify

We have a message if we want to add items and we can't add, substract only.
It's ok if we choose to create items on ordering, in this case koha can't add items, just substract
and in this case we have to delete manually the items(s) in the catalog.
But if via the syspref AcqCreateItem we choose to create items when receiving this limitation is not usefull
The patch just checks if the syspref AcqCreateItem is on 'ordering'
if not the message is not shown and we can add items

Signed-off-by: Mathieu Saby <mathieu.saby@univ-rennes2.fr>
Here is the test I made for signing off :
after applying the patch
- syspref AcqCreateItem  : create items on RECEIVING
- in a basket, create an order (quantity = 1)
- save the order
- reopen the order
- change the quantity (2 instead of 1)
- save the order
=> changing quantity was not possible before the patch

- syspref AcqCreateItem  : create items on CATALOGING
- in a basket, create an order (quantity = 1)
- save the order
- reopen the order
- change the quantity (2 instead of 1)
- save the order
=> changing quantity was not possible before the patch

- syspref AcqCreateItem  : create items on ORDERING
- in a basket, create an order (click on "add" to add an item => quantity = 1)
- save the order
- reopen the order
- try to change the quantity (2 instead of 1), without clicking on "add" to create a new item => you cannot (alert message)
=> the behavior is the same as before the patch

Signed-off-by: Katrin Fischer <Katrin.Fischer.83@web.de>
Copied test plan from bug report.
Template only change deactivating the Javascript that blocks
you from changing the quantity when AcqCreateItem is set to
something else than 'ordering'.
Passes all tests and QA script.
**Rebased for 3.8.x**

Signed-off-by: Chris Cormack <chris@bigballofwax.co.nz>
Bug 9923 - correction of a bug due to bz 7129

See comments in the patch

Signed-off-by: Mathieu Saby <mathieu.saby@univ-rennes2.fr>
Signed-off-by: Paul Poulain <paul.poulain@biblibre.com>

**rebased for 3.8.x**

Signed-off-by: Chris Cormack <chris@bigballofwax.co.nz>