The course_visiblity field can have one of three values:
1. private (default): This keeps the standard access rules.
2. public_outline: Allows unenrolled and anonymous users access to the outline.
3. public: Allows unenrolled and anonymous users access to both outline and
   course content.

When an unenrolled user accesses course content, instead of student_view(),
public_view() is used. A default implementation is provided for XBlocks
which do not implement this view. The public_view() must not have any
functionality which assumes the presence of a valid User and should show
a readonly only interface for the XBlock content.