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

https://openedx.atlassian.net/browse/MA-184

@gwprice @aleffert Please review.
@gwprice I don't see where the Course-About information is currently used on the app.  @aleffert Have you see it anywhere?  I intentionally left the course-about endpoint as is since we allow users to see information about an upcoming course before it's released.  If the course-author chose to "publish" the course, I would think they would want to present its "about" info.