Bug 24257: Fix close of modal on 'Yes, print slip'

Bug 24257: Fix close of modal on 'Yes, print slip'

When a item-transfer-modal is prompted by returns.tt and the option
"Yes, print slip" is selected the transfer is not initiated and the
modal does not disappear.

The print_slip hidden input must be in the same form as the
submit/button .print, otherwise the browser logs
  this.form.print_slip is undefined

Test plan:
1. set AutomaticItemReturn to "don't"
2. Check out an item and attempt to check it in at a branch it doesn't
belong to.
3. When the popup modal appears select 'Yes, print slip'.
=> Observe that the transfer does initiate and the modal closes

Signed-off-by: Lucas Gass <lucas@bywatersolutions.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>