LONDON (Reuters) -British Prime Minister Boris Johnson was fighting on Friday to shore up his authority after a senior aide resigned over his false claim that the leader of the opposition Labour Party failed to prosecute a notorious child sex abuser. Johnson, who in 2019 won the biggest Conservative majority since Margaret Thatcher, has repeatedly refused to resign over reports that he and some of his staff attended Downing Street parties during COVID-19 lockdowns. Those revelations raised questions about Johnson’s often chaotic style of leadership and have led to the greatest threat to him si…