Spirit Airlines has operated for nearly 50 years, and its ultra-low-cost fares pushed prices down on many routes. Kevin Carter/Getty Images The ultra-low-cost airline announced it is shutting down after nearly 50 years. A proposed bailout fell through. The US government offered $500 million in exchange for a 90% stake. Spirit said it would automatically process refunds for flights bought with a credit or debit card. After two bankruptcies and a failed federal bailout, Spirit Airlines is out...