In more than 53 years of flying, Southwest Airlines never charged for bags, never had a mass layoff, and never assigned seats. Well, never say never again, because all of that has now happened—or is about to happen—at the Dallas-based airline.On Tuesday, Southwest announced it will end its “Bags Fly Free” policy, which allowed every passenger to check two bags at no charge. The company also announced a new, nonrefundable basic-economy fare class, with more restrictions. Both changes take effect May 28.These moves are the latest in a shake-up that’s been rattling Southwest since last summer, when activist investor Elliot Investment Management took a big stake in the carrier and pushed for change. Since then, Southwest has decided to end its long-standing open-seating policy in…