add speed_grader_url to the quizzes api

Test plan:
  - As a teacher, you should be able to see "speed_grader_url" that
    goes to the correct url when you query the quizzes index api and the
    quiz show api.
  - As a student, you should not see the "speed_grader_url" key when
    you query the quizzes index api and the quiz show api.

Change-Id: I9a827846fcf201d031715567e8af1f6ffda4a343
Reviewed-on: https://gerrit.instructure.com/30127
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Caleb Guanzon <cguanzon@instructure.com>
Reviewed-by: Jason Madsen <jmadsen@instructure.com>
Product-Review: Stanley Stuart <stanley@instructure.com>