Bug 23673: Add "Updated on" column to patron's notices

Bug 23673: Add "Updated on" column to patron's notices

This patch adds "Updated on" column to patron's notices tab. It also adds logic to C4::Letters to retrieve updated_on column.

To test:

1. Apply patches.
2. Restart plack.
3. Choose a patron and add a purchase suggestion.
4. Change suggestion status.
5. Open patron's notifications.
   CHECK => Messages table has now "Updated on" and "Time created" columns, and "Time" column is gone.
   SUCCESS => There is a message with status pending, with a "time created" that equals "updated on"
6. Execute in the shell in Koha directory
   $ ./misc/cronjobs/process_message_queue.pl
7. Open patron's notifications one more time.
   SUCCESS => The message changed status. Time created remained the same, and now "updated on" has the current timestamp.
8. Resend the message and repeat sep 6.
   SUCCESS => Every time you change the status, time created remains the same and updated on updates.
9. Run `prove t/db_dependant/Letters.t`
10. Sign off

Signed-off-by: Kelly McElligott <kelly@bywatersolutions.com>
Signed-off-by: Andrew Fuerste-Henry <andrew@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>