AMC Boston Common 19 is no longer packed with throngs of teens or awkward first daters. The newly reopened complex can fit thousands of people in front of its 19 screens, but when you walk in now, you’ll likely find that it’s just you and two employees. They’ll scan your ticket from behind plexiglass shields and give you a funny look. They’re surprised to see you. When Covid-19 hit, movie theaters closed overnight. It was obvious they had to. Scientists told us to mask up and barricade ourselves at home. Squeezing between heavy-breathing strangers for two hours? It sounded like a death trap. But by deciding that movie theaters were…