Logistration: Auto-enroll fails after registering a new account

[ECOM-622](https://openedx.atlassian.net/browse/ECOM-622):  Auto-enroll fails after registering a new account.

The underlying issue is that Django rest framework's session authentication requires that a user is logged in with an *active* account.  Yet we want users to be able to enroll in courses as soon as they register.

The fix removes the check for account activation for the change enrollment end-point in the enrollment API.  I think this is safe because the end-point allows users to modify only their own enrollments.

Reviewers: @stephensanchez @dianakhuang