Jem Aswad Senior Music EditorIn the wake of British Prime Minister Boris Johnson’s announcement last month that mass-gatherings can begin a cautious return this summer, the London music festival All Points East has announced more performers for its 2021 edition — scheduled for August 27-30 in Victoria Park — and the country’s largest festival, Glastonbury, is moving ahead with plans for a scaled-down event in September.Joining previously announced All Points East headliners Jamie xx, Kano, and