Bug 22421: (follow-up) Use checkout accessor to get correct issue_id

Bug 22421: (follow-up) Use checkout accessor to get correct issue_id

In some cases, we may want to update an AccountLine that is linked to a
checkout without knowing whether it is a current or old checkout. In
these cases, use the AccountLines ->checkout accessor to ensure we get
the issue_id correctly, regardless of whether it is a current issue or
and old_issue.

This also applies when generating a new AccountLine based on an existing
AccountLine (for example, adding a credit to refund a previously charged
fine)

Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>