Clear state option

To fix the buggy behavior reported in TNL-3880, we need to ensure
that the submission we're dropping is unlinked from the student_item
used to find it. This prevents code (say, an ORA staff tool) from
constructing a student item with an id, a course, and a problem, and
using that to find a "cleared" submission.

submissions/api.py

@@ -643,7 +643,7 @@ def get_latest_score_for_submission(submission_uuid, read_replica=False):
     return ScoreSerializer(score).data
 
 
-def reset_score(student_id, course_id, item_id):
+def reset_score(student_id, course_id, item_id, clear_state=False):
     """
     Reset scores for a specific student on a specific problem.
 
@@ -655,6 +655,7 @@ def reset_score(student_id, course_id, item_id):
         student_id (unicode): The ID of the student for whom to reset scores.
         course_id (unicode): The ID of the course containing the item to reset.
         item_id (unicode): The ID of the item for which to reset scores.
+        clear_state (boolean): If True, unlink the Submission and StudentItem so the Submission cannot be accessed.
 
     Returns:
         None
@@ -684,6 +685,11 @@ def reset_score(student_id, course_id, item_id):
             item_id=item_id,
         )
 
+        if clear_state:
+            # sever the link between this student item and any submissions it may current have
+            for sub in student_item.submission_set.all():
+                sub.student_item = None
+
     except DatabaseError:
         msg = (
             u"Error occurred while reseting scores for"