Translateded missing entries "ThirdPartiesOfSaleRepresentative"

Added and translateded missing entries:

- /langs/it_IT/commercial.lang - ThirdPartiesOfSaleRepresentative
- /langs/it_IT/main.lang - LinkedToSpecificUsers

Affected pages are (HTTP prefix, server and path were stripped):

- compta/facture/list.php?leftmenu=customers_bills