Bug 17969: Refactor the way <<items.content>> is generated

Bug 17969: Refactor the way <<items.content>> is generated

<<items.content>> is generated 4x in advance_notices.pl and once in
overdue_notices.pl
It would be better to have it in C4::Letters.
It will enforce the fact that it already has the same behavior, make it
testable and reusable.

Test plan:
Use the <<items.content>> tag for advance and overdue notices.
The generated notices must be the same as before this patch.

Followed test plan, works as expected.
Signed-off-by: Marc Véron <veron@veron.ch>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>