Bug 30979: Add checkout modal to the OPAC

Bug 30979: Add checkout modal to the OPAC

This patch adds a new self checkout modal to the OPAC when
OpacTrustedCheckout is enabled and a user is logged in.

The new modal allows an end user to scan an item barcode to checkout.
We check for item existance and availability and then check the item
out after any confirmations have been displayed.

Signed-off-by: Silvia Meakins <smeakins@eso.org>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>