Quiz Stats - don't count Student View submissions

QuizSubmission now supports a new scope "for_students" that will grab
all submissions that are:

  - not a teacher preview
  - not a teacher-initiated "Student View" one
  - submitted by students who really are rolled in a course

Closes CNVS-13694

TEST PLAN
---- ----

  - create a quiz with one question
  - enter Student View via the course settings
    + take the quiz
    + get out of Student View
  - visit the statistics page (or hit the API)
    + verify the attempt is not factored
  - generate the CSV of Item-Analysis to verify that IA does not factor
    these attempts either
  - test the change with Survey quizzes - need to make sure "logged out"
    users are still OK where applicable

Change-Id: Ic58569be31576734eb6e309d548a7415c07e08a7
Reviewed-on: https://gerrit.instructure.com/38174
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jason Madsen <jmadsen@instructure.com>
Product-Review: Ahmad Amireh <ahmad@instructure.com>
QA-Review: Caleb Guanzon <cguanzon@instructure.com>