Hope Hicks, the White House communications director and one of President Trump’s longest-serving advisers, said Wednesday that she was resigning. Ms. Hicks, 29, a former model who joined Mr. Trump’s 2016 presidential campaign without any experience in politics, became known as one of the few aides who understood his personality and style and could challenge the president to change his views. Ms. Hicks had been considering leaving for several months. She told colleagues that she had accomplished what she felt she could with a job that made her one of the most powerful people in Washington, and that there would...