There aren’t many rules when it comes to selling commercials , but among traditional media companies, one has long been honored: Before the new head of a sales division comes to work, the predecessor usually leaves. Paramount Skydance is taking a different tack. The company, led by entrepreneur David Ellison, last week unveiled an unorthodox new structure for its ad sales operations. Jay Askinasi ( above, pictured , left ), formerly a senior sales executive at Roku, will arrive November 3 as