Adds CouseModule.course_visibility and XBlock.public_view() for unenrolled users access to courses.

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.

common/lib/xmodule/xmodule/html_module.py

@@ -79,6 +79,13 @@ class HtmlBlock(object):
         """
         return Fragment(self.get_html())
 
+    @XBlock.supports("multi_device")
+    def public_view(self, context):
+        """
+        Returns a fragment that contains the html for the preview view
+        """
+        return self.student_view(context)
+
     def student_view_data(self, context=None):  # pylint: disable=unused-argument
         """
         Return a JSON representation of the student_view of this XBlock.