"As explained above, the murder-for-hire evidence is probative both as evidence of the charged offenses and to prove Ulbricht’s identity as DPR—a key disputed issue in this case. In addition, the charges in this case are extremely serious: Ulbricht is charged not with participating in a run-of-the-mill drug distribution conspiracy, but with designing and operating an online criminal enterprise of enormous scope, worldwide reach, and capacity to generate tens of millions of dollars in commissions."