LOS ANGELES—On a crisp November evening three weeks after the election, about 600 local activists gathered on a sound stage at the old Mack Sennett studios in Silver Lake where nearly a century ago the Keystone Cops slipped on banana peels and dodged pies in the face. These avatars of Hollywood liberalism had just suffered a similar humiliation and had come to figure out how to pick themselves up and start all over again in Donald Trump’s America.