add excused? to the quiz_submission api

fixes CNVS-22706

test plan:
- go to api/v1/courses/:course_id/quizzes/:quiz_id/submissions
  and ensure that excused? is there

Change-Id: I63ed16ad46f3af20357ff95c8900e32d94b0c108
Reviewed-on: https://gerrit.instructure.com/66508
Tested-by: Jenkins
Reviewed-by: Ryan Taylor <rtaylor@instructure.com>
QA-Review: Landon Holmstead <lholmstead@instructure.com>
Product-Review: Davis McClellan <dmcclellan@instructure.com>

lib/api/v1/quiz_submission.rb

@@ -49,6 +49,7 @@ module Api::V1::QuizSubmission
     time_spent
     attempts_left
     overdue_and_needs_submission
+    excused?
   ].freeze
 
   def quiz_submission_json(qs, quiz, user, session, context = nil)