Bug 18626: Add ability to track cardnumber changes for patrons

Bug 18626: Add ability to track cardnumber changes for patrons

Some libraries want to keep a history of cardnumber changes for a
patron. This is a feature that another ILS has.

Test Plan:
1) Apply this patch
2) Enable the BorrowersLog
3) Modify a patron, update his or her cardnumber
4) Notice the entry in the action log containing the old and new cardnumbers
5) Edit a patron, leave the cardnumber the same
6) Note there is no cardnumber change entry in the action logs
7) Disable BorrowersLog, modify a patron cardnumber
8) Note there is no cardnumber change entry in teh action logs

Followed test plan, works as expected
Signed-off-by: Marc Véron <veron@veron.ch>
Signed-off-by: Josef Moravec <josef.moravec@gmail.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>