[IMP] stock: don't auto. assign backorder

Before this commit, when a backorder is created, it was automatically
reserved. It is no more the case. Now user have to reserve it manually.

The reason is, when a user doesn't delivery all expected quantities, it
could be because there is no enough quantity in stock, or the user
doesn't want to delivery all. In these cases, it doesn't make sense to
reserved it directly.

task-2181611