“Ruling in a case involving General Motors LLC, the National Labor Relations Board has given private-sector employers more leeway to dole out discipline or fire workers for racist, sexist, and other offensive speech or conduct in the context of organizing or some other union-related activity. The Tuesday decision reinstates a 40-year-old test that had been supplanted by a series of setting-specific rulings that previously had applied to conduct or speech during encounters with management, on picket lines, and on social media.”