Bug 25596: Identify overpayment refunds for translation

Bug 25596: Identify overpayment refunds for translation

This patch ensures we can uniquely identify automatic overpayment refund
credits from the accountlines by introducing a new account credit type
and adding appropriate translations for the new code.

Test plan
1/ You'll need an overpayment refund to start...
1a/ Issue yourself and item and set the date due early enough
to create an overdue fine for at least 2 days worth of overdue fines
1b/ Run the fines.pl script to add the fines
1c/ As a patron, pay off the accruing fine
1d/ Return the item ensuring you backdate it at least a day
1e/ Check that there is an 'Overpayment refund' credit on the patrons
account.
2/ Check that the overpayment string is translatable (i.e. it appears in
the accounts include file)

Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Caroline Cyr La Rose <caroline.cyr-la-rose@inlibro.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>