Bug 24083: Add support for unseen_renewals

Bug 24083: Add support for unseen_renewals

This patch adds support for unseen renewals.

Here we retrofit knowledge of unseen renewals and add the display of unseen
renewal counts and warnings, in addition to adding the ability to
specify a renewal as being "unseen".

The functionality added here is goverened by the UnseenRenewals syspref.

Signed-off-by: Sally Healey <sally.Healey@cheshirewestandchester.gov.uk>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>