A Randomized, Controlled, Open-Label, Rater-Blinded, Phase 3b Study of the Efficacy, Safety, and Tolerability of 6-Week Extended Interval Dosing of Natalizumab (BG00002) in Subjects With Relapsing-Remitting Multiple Sclerosis Switching from Treatment with 4-Week Natalizumab Standard Interval Dosing (SID) in Relation to Continued SID Treatment Followed by an Open-Label Crossover Extension Study Comprising Subcutaneous and Intravenous Natalizumab Administration