Update mode appropriately when user enrolls

The "mode" was remaining unchanged when a student unenrolled & re-enrolled in a course--i.e., someone who was verified when first registering would still be verified when unenrolling and re-registering in audit mode.  This commit fixes this problem.

LMS-1526
Merge pull request #1771 from edx/flowerhack/fix/lms-1526

Update mode appropriately when user enrolls