Bug 20688: Add accesskeys to hold modals

Bug 20688: Add accesskeys to hold modals

This patch adds accesskey elements to the hold modals during checkin

For tips on how to access the accesskeys:
https://developer.mozilla.org/en-US/docs/Web/HTML/Global_attributes/accesskey

To test:
1 - Place some holds on items
2 - Check in an item with a hold at a branch that requires transfer
3 - Use accesskey I to ignore
4 - Use accesskey Y to confirm and transfer
5 - Check in again
6 - Use accesskey Y to confirm
7 - Repeat 4-6 with a new hold but use the accesskey P to print the slip
8 - Check in the item again ans use accesskey X to cancel

Signed-off-by: Owen Leonard <oleonard@myacpl.org>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>