Social media has been bursting with photos of crowded planes amid the pandemic. That raises the question of how it’s even possible when air travel is down more than 90% from a year ago. In some cases, airlines are creating the crowds by canceling other flights and packing passengers onto fewer planes. But the carriers say they are taking action to ease passengers’ fears about coronavirus contagion. Some are blocking middle seats or letting passengers pay extra to guarantee an empty seat next to them. They are also starting to require passengers to wear facial coverings.