PLYMOUTH, Mass. — The seaside town where the Pilgrims came ashore in 1620 is gearing up for a 400th birthday bash, and everyone’s invited — especially the native people whose ancestors wound up losing their land and lives. Plymouth, Massachusetts, whose European settlers have come to symbolize American liberty and grit, marks its quadricentennial in 2020 with a trans-Atlantic commemoration that will put Native Americans’ unvarnished side of the story on full display. “It’s history.