Only set due dates on graded content

When a self paced course was published and we were configured to
automatically space out due dates among its sections, we were
accidentally setting due dates on ungraded content. Which limited
access to it.

AA-162

openedx/core/djangoapps/course_date_signals/handlers.py

@@ -58,9 +58,10 @@ def extract_dates_from_course(course):
                 items = [section]
                 while items:
                     next_item = items.pop()
-                    # TODO: Once studio can manually set relative dates,
-                    # we would need to manually check for them here
-                    date_items.append((next_item.location, {'due': time_per_week * (idx + 1)}))
+                    if next_item.graded:
+                        # TODO: Once studio can manually set relative dates,
+                        # we would need to manually check for them here
+                        date_items.append((next_item.location, {'due': time_per_week * (idx + 1)}))
                     items.extend(next_item.get_children())
     else:
         date_items = []