Pick the latest visible course instance in the redirect

If the course has only hidden instances and the user is
course staff, then the latest hidden instance is used in the
redirect. Otherwise, the latest visible instance is always used.

Fix #791

course/views.py

@@ -113,11 +113,22 @@ def get_resource_objects(self):
         course = get_object_or_404(Course, url=self._get_kwarg(self.course_kw))
         course_instances = CourseInstance.objects.filter(course=course).order_by('-starting_time')
 
-        if course_instances:
-            self.course_instance = course_instances[0]
+        latest_visible = None
+        latest_hidden = None
+        for instance in course_instances:
+            if instance.visible_to_students:
+                latest_visible = instance
+                break
+            else:
+                latest_hidden = latest_hidden or instance
+
+        if latest_visible:
+            self.course_instance = latest_visible
+        elif latest_hidden and latest_hidden.is_course_staff(self.request.user):
+            self.course_instance = latest_hidden
         else:
             raise Http404(_("There are no course instances for this course."))
-    
+
     def get(self, request, *args, **kwargs):
         return self.redirect(self.course_instance.url)