PHILADELPHIA — For months there were warnings that if the presidential election dragged on past Election Day, Philadelphia could be the site of mass civil unrest or political violence.But something very different happened, instead. The city danced.As the world’s eyes were on the largest city in the most critical swing state, Philadelphians sang and protested and marched and pasted printouts of Gritty in weird places. Videos of people grooving while in line to cast their mail ballot went viral before Election Day, then again when hundreds took part in a dance party for two days outside the Conv…