Prohibit automatic enrollment in prof ed courses when user authenticates via third-party auth

@stephensanchez @dianakhuang @wedaly, as discussed.
I like that, I think it's a little clearer than the current logic. The current conditional mirrors the conditional in the [student views](https://github.com/edx/edx-platform/blob/master/common/djangoapps/student/views.py#L808) - would you still encourage me to change it?