Matthew Taylor, a conceptual artist and political strategist, made influential ads and documentaries produced by Steve Bannon. Now he’s assisting the White House.Matthew Taylor officially holds one role in the Trump administration as a senior adviser at the National Endowment for the Humanities, but he has quietly become a key player behind many of the president’s bold initiatives to transform Washington and its cultural legacy.This year, Mr. Trump appointed Mr. Taylor to the Commission of Fine Arts, a federal agency that reviews government construction in Washington, to rubber-stamp approvals for projects such as the White House ballroom and the world’s...