By Nandita Bose and Kanishka Singh EMMAUS, Pennsylvania (Reuters) -U.S. President Joe Biden on Friday called Yemen’s Houthi forces a “terrorist” group, after American and British warplanes, ships and submarines launched dozens of air strikes across Yemen overnight. Biden said Washington will respond to the Houthis if they continued behavior that he called “outrageous.” WHY IT IS IMPORTANT London and Washington say their strikes in Yemen were in retaliation for months of attacks by the movement on Red Sea shipping that the Iran-backed fighters cast as a response to the ongoing war in Gaza. The …