SACRAMENTO, Calif. — President Trump is vowing a new approach to getting homeless people off the streets by forcibly moving those living outside into large camps while mandating mental health and addiction treatment — an aggressive departure from the nation's leading homelessness policy, which for decades has prioritized housing as the most effective way to combat the crisis. "Our once-great cities have become unlivable, unsanitary nightmares," Mr. Trump said in a presidential campaign video. "For those who are severely mentally ill and deeply disturbed, we will bring them to mental institutions, where they belong, with the goal of reintegrating them...