What does toilet paper have to do with a global pandemic? Nothing. Yet millions of people have been panicking about their household supply. Stores shelves have been emptied. Amazon is often out of stock. And social media is bursting with jokes and pleas for a roll or two. The good news: Things are calming down, at least in the U.S., after a buying spree in mid-March. But it’s not yet clear when — if ever — buying habits will get back to normal.