On September 17, 2001, the great David Letterman became the first late night host to return to television days after 9/11 blindsided New York City, and the world. In his monologue, Letterman eschewed attempts at humor, choosing instead to meet the confusion and mourning blanketing America with an appropriate solemn acknowledgment. This became the pause the nation needed before feeling permission to laugh again in the wake of a horrific tragedy. Nearly 20 years later another great Dave — Dave Chappelle — has handed a different permission slip to Americans : one giving folks the green light to laugh at tragedies and, more to the point, the victims and survivors. Maybe that’s not being fair. The 9/11 attacks resulted in the deaths of 2,996 people and the injuries of more than 6,000 others — a massive culture-shifting calamity by any feeling person’s definition.…