Saudi Arabia’s King Salman has appointed his son, Mohammed bin Salman, as crown prince, replacing his nephew, Mohammed bin Nayef, as first in line to the throne. WSJ’s Niki Blasina explains who he is, and what this means for the U.S., Saudi Arabia and the broader region. Photo: Getty Images. The new heir to Saudi Arabia’s throne has launched a crackdown on dissent in recent weeks, attempting to silence activists and critical clerics as well as his deposed predecessor, according to U.S. and Saudi officials familiar with the events. King Salman upended Saudi Arabia’s succession order last month by naming his 31-year-old son, Mohammed bin Salman, crown prince and next in line to the throne, and sidelining his nephew and…