The certification of the plaintiff class was not consistent with Rule 23(a)...[which] requires a party seeking class certification to prove that the class has common “questions of law or fact.” Their claims must depend upon a common contention of such a nature that it is capable of classwide resolution—which means that determination of its truth or falsity will resolve an issue that is central to the validity of each one of the claims in one stroke...Respondents’ backpay claims were improperly certified under Rule 23(b)(2)...[which] applies only when a single, indivisible remedy would provide relief to each class member.