Fast-food workers have launched a historic series of strikes as part of an escalating nationwide call for the right to unionize and earn wages of $15 an hour, more than double the current federal minimum wage. Actions are taking place this week in at least seven cities, including Chicago, St. Louis, Detroit, Kansas City and Milwaukee, as part of a coordinated campaign by workers in the largely non-union industry. In New York City on Monday, hundreds of employees from McDonald’s, Burger King, Wendy’s and other chains walked off the job for the day and rallied, chanting, “We can’t survive on 7.25.”