The email was cordial, warm and deferential. “Thank you so much for the extremely kind invitation to play at your inauguration,” wrote one of President Trump’s favorite musicians, Sir Elton John. “I have given it at lot of thought, and as a British National I don’t feel that it’s appropriate for me to play at the inauguration of an American President. Please accept my apologies.” Mr. Trump had been hoping Mr. John would croon him into the...