feat(graph): implement lastSubmission

Author: pan93412

graph/question.graphqls

@@ -64,10 +64,15 @@ extend type Question {
   referenceAnswerResult: SQLExecutionResult! @scope(scope: "question:read")
 
   """
-  List of your submissions for this question.
+  List of your submissions for this question, ordered by submitted at descending.
   """
   userSubmissions: [Submission!]!
 
+  """
+  Get the last submission for this question.
+  """
+  lastSubmission: Submission
+
   """
   Have you tried to solve the question?
   """