BUG 5607 : Adds parsing of issues fields in overdue letters

BUG 5607 : Adds parsing of issues fields in overdue letters

Signed-off-by: Katrin Fischer <Katrin.Fischer.83@web.de>
Patch makes it possible to add fields from the issues table to overdue notices.

Template used for testing:
<item>"<<biblio.title>>" by <<biblio.author>>, <<items.itemcallnumber>>, Barcode: <<items.barcode>> , Checkout date: <<issues.issuedate>>, Due date: <<issues.date_due>> Fine: <fine>GBP</fine> Checkout date from items: <<items.onloan>></item>

Possible improvements:
- Dates are not formatted according to dateformat system preference

Signed-off-by: Paul Poulain <paul.poulain@biblibre.com>