Added item_db2.sql conversion script to the new structure

- For those using a SQL item_db2, I provided a convenience script to
  update to the new structure without having to delete and re-import
  your data.
- The script requires at least MySQL 5.0 (tested on MySQL 5.6) and works
  for both Pre-Renewal and Renewal tables. If you're running an older
  version (Oracle advises to upgrade for security reasons by the way!),
  open the upgrade script in your text editor, and you'll find
  the queries you can run manually instead.
- Special thanks to Ind.

Signed-off-by: Haru <haru@dotalux.com>