Bug 21978: Add middle_name field to the database

Bug 21978: Add middle_name field to the database

This patch adds a new field, middle_name, to the borrowers,
deletedborrowers and borrower_modifications tables.

It also updates the DefaultPatronSearchFields preference to include the
new field if the preference is still set to it's default settings from
install.

Sponsored-by: Cheshire Libraries
Signed-off-by: Sally <sally.healey@cheshiresharedservices.gov.uk>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>