A Multicenter, Randomized, Double-blind, 2-Part Phase 2 Study to Evaluate the Efficacy and Safety of GS 1427 in Adult Participants With Moderately to Severely Active Ulcerative Colitis (UC) Part 1: Placebo-Controlled, Dose-Ranging Study of GS 1427; Part 2: Active-Controlled, Combination Study Evaluating the Efficacy and Safety of GS-1427 in Combination With Ustekinumab versus GS-1427 or Ustekinumab monotherapy