President Trump tweets most mornings, but Wednesday was different. In addition to doling out the usual kind of fare about CNN, the stock market, and North Korea, the president, or whoever was tweeting on his behalf, also decided to retweet three videos from an account belonging to a leader of a far-right ultranationalist British political group “Britain First.” The unverified videos purport to show Muslims committing acts of violence, from beating up a boy on crutches to destroying a statue of the Virgin Mary. One depicts a mob pushing someone off a roof.