WASHINGTON — Thousands of people wanting to send a message that racism is unwelcome in the United States gathered in a park outside the White House to protest a white nationalist rally on the anniversary of the deadly violence in Charlottesville, Virginia. In the end, fewer than two dozen white nationalists showed up. The events held in both Charlottesville and Washington, largely peaceful though tense at times, were part of a day of speeches, vigils and marches marking a year since one of the largest gatherings of white nationalists and other far-right extremists in a decade.