Bug 27847: Don't obscure page when checkin modal is non-blocking

Bug 27847: Don't obscure page when checkin modal is non-blocking

This patch changes the series of events striggered when a non-blocking
modal dialog is displayed during checkin. If the "TransfersBlockCirc"
preference is set to "Don't block," the user can continue to check in
even though a transfer dialog has appeared. This patch hides the modal's
"backdrop" element so that the modal doesn't look like one which blocks
further action.

There is no Bootstrap backdrop option which both hides the backdrop and
makes it non-blocking, so we have to use the default non-blocking option
and "manually" hide the backdrop.

The "show.bs.modal" event must be added in order to make the
necessary change before the modal is displayed. This prevents the
darkened backdrop from appearing and then being hidden.

To test, apply the patch and rebuild the staff interface CSS
(https://wiki.koha-community.org/wiki/Working_with_SCSS_in_the_OPAC_and_staff_client).

- Set the "TransfersBlockCirc" preference to "Don't block."
- Check in an item which belongs to another library.
- A modal dialog should be displayed: "Please return this item to XXX"
- The page "behind" the modal should not be darkened, and the cursor
  focus should remain in the check-in field.
- Checking in an item which as a hold should still trigger the
  "blocking" behavior: The modal must be dismissed by clicking one of
  its buttons (not the backdrop) and the page behind the modal should be
  darkend.
- Test with "TransfersBlockCirc" set to "Block." Checking in an item
  belonging to another library should trigger the "blocking" modal
  behavior.
- Test that other checkins still work correctly.

Signed-off-by: Lucas Gass <lucas@bywatersolutions.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>