Bug 29100: (QA follow-up) Rename issue(s) keys to checkout(s)

Bug 29100: (QA follow-up) Rename issue(s) keys to checkout(s)

Adapted test plan:

1) Apply this patch
2) For DUE and PREDUE notices, set the message body to the following:
   Title: [% checkout.title %]
3) For DUEDGST and PREDUEDGST notices, set the message body to the following:
   Titles:
   [% FOREACH c IN checkouts %]
    * [% c.title %][% END %]
4) Generate PREDUE and DUE notices for patrons including digests
5) Verify those notices contain the checkout titles

Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Edit: tcohen renamed @issues => @checkouts as well
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Tomás Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>