Rent has never been less affordable — for tenants with high and low incomes alike — even while costs for new leases are finally cooling off.Half of American renters spend more than 30 percent of their income on housing costs — a key benchmark for affordability — with the financial strain rising the fastest for middle-class tenants. That’s according to a new report from the Harvard Joint Center for Housing Studies, which found that the number of such renters, considered to be “cost-burdened,” hit...