Make hidden courses accessible with LTI

When an A+ course is intended to be accessed as LTI tool from another
learning platform, teacher may wish to prevent direct access from
the regular A+ front page, for example to avoid possible confusion in
grade management  between the two systems. Therefore teacher can now
hide a course, while it is still available with LTI.

Closes #1240

course/permissions.py

@@ -137,7 +137,8 @@ def is_object_visible(self, request, view, course): # pylint: disable=arguments-
             return True
 
         # Course is not visible if it's hidden
-        if not course.visible_to_students:
+        # If course is launched through LTI, it is shown even if marked as hidden in A+ UI.
+        if not request.session.get('lti-launch-id') and not course.visible_to_students:
             self.error_msg(_('COURSE_VISIBILITY_ERROR_NOT_VISIBLE'))
             return False