Skip audit enrollment if user is in the experimental branch

This addresses an issue which caused a user in the experimental branch to be enrolled as audit mode instead of honor mode when selecting the "Pursue Honor Code Certificate" option on track selection page. @wedaly and @dianakhuang, could you please review this?
@flowerhack The way Optimizely works, once you're in the experimental group, you're in the experimental for all courses with a verified track.