Bugfix for ECOM-199: Visitors removed from auto-enroll pool if they need to authenticate while attempting to enroll
Sets a session variable as soon as a user enters the experimental condition to ensure that they stay in that condition,
even if they later access the control URL (e.g. from a redirect on the login page)
Exclude courses from the auto-registration A/B test
Merge pull request #5006 from edx/will/ecom-199

ECOM-199: Auto-registration bugfix