There’s nothing like a sold-out arena gig to remind you that online discourse is only a small part of any artist’s story. Viewed from behind a screen, Arctic Monkeys’ headline set at Glastonbury this year appeared divisive. ‘The new songs are boring’, ‘Alex Turner’s affectations have gone too far’, ‘they’ve lost their mojo’. Try telling that to the thousands of adoring fans inside a sold-out arena tonight. The crowd is young – astonishingly so for a band with nearly two decades under its belt and seven albums, the last two of which have been daringly, even deliberately uncommercial. The Arctic…