Bill de Blasio is summering in Italy. And visiting Northern Ireland too. Five years out of office, the former mayor of the greatest city in the world has been sharing his hard-earned lessons in various corners of Europe. That meant headlining a May event in Perugia with Italian Partito Democratico leader Elly Schlein – then seeing a mural unveiled at the Port of Naples, which included a depiction of his own grandparents. That also meant speaking…