If only it were so easy: Put together a prom and open people’s hearts.The most beautiful thing about Broadway musical “The Prom” — and there are a lot of beautiful things about “The Prom” — is that for a shining moment of theatrical magic, this uplifting, heart-filled show makes you believe that it’s possible: With some balloons, crepe-paper streamers, a punch bowl and a DJ, you can change the world.