MA-184 Prevent student access to unreleased data through mobile API

https://openedx.atlassian.net/browse/MA-184

@gwprice @aleffert Please review.
I'm also wondering whether it's necessary here: https://github.com/edx/edx-platform/blob/mobile/MA-184/lms/djangoapps/mobile_api/course_info/views.py#L117