Don't assume due dates on sections

The content highlights code assumed due dates existed on all
sections. But we recently broke that assumption. So now we
recalculate the spread of sections across the expected duration
ourselves rather than rely on due dates.

openedx/features/course_duration_limits/access.py

@@ -67,6 +67,10 @@ def get_user_course_duration(user, course):
     if enrollment is None or enrollment.mode != CourseMode.AUDIT:
         return None
 
+    verified_mode = CourseMode.verified_mode_for_course(course=course, include_expired=True)
+    if not verified_mode:
+        return None
+
     return get_expected_duration(course)