Feedback shortPolling / statistics / +minor fixes

Student names in Subscription list for reviewer

Short polling on orig Feedback

Refresh button student list

FeedbackComment visiblity indication

SubscriptionEnded page

Right side of student overview is sticky

Statistics overview

core/serializers/feedback.py

@@ -141,7 +141,10 @@ class FeedbackSerializer(DynamicFieldsModelSerializer):
 
         has_full_score = score == submission.type.full_score
         has_feedback_lines = ('feedback_lines' in data and
-                              len(data['feedback_lines']) > 0)
+                              len(data['feedback_lines']) > 0 or
+                              self.instance is not None and
+                              self.instance.feedback_lines.count() > 0)
+
         if not has_full_score and not has_feedback_lines:
             raise serializers.ValidationError(
                 'Sorry, you have to explain why this does not get full score')