Bug 14368: Add discharges history.

Bug 14368: Add discharges history.

Currently, once a user is discharged, lifting the discharge debarment does
not allow the user to ask for a discharge again.

This patch adds the ability for a user to be discharged several times and to
keep an history of the discharges.

Test plan:

 - As a user, ask for a discharge.
 - As a librarian, allow the discharge.
 - Check that the requested and validated dates appear on the discharge panel of the borrower in the admin interface.
 - Lift the discharge debarment of the user.
 - Check that you can repeat these steps and that the new discharges are added in the discharge panel of the borrower in the admin interface.

Please note that you will need to run misc/devel/update_dbix_class_files.pl after the SQL update.
Adding a primary key to the discharges table was mandatory for DBIX::Class to be able to query with order_by.

Signed-off-by: Indranil Das Gupta (L2C2 Technologies) <indradg@gmail.com>
Signed-off-by: Jonathan Druart <jonathan.druart@koha-community.org>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Tomas Cohen Arazi <tomascohen@unc.edu.ar>