Stop counting teacher previews in results

The scope QuizSubmission#logged_out now excludes teacher previews. This
should solve all occurrences of logged-out users mixing up with teacher
preview submissions.

Closes CNVS-7851

TEST PLAN
---- ----

  - create a quiz
  - preview it, submit your attempt
    - click on the "Show Student Quiz Results" and verify that your
      preview attempt does not show up
  - create a survey quiz and verify that we still get to see the
    "Logged Out User" submissions correctly
  - verify we don't have any regressions on quiz stats with teacher
    previews (a simple API should suffice, they're now configured to
    share the same code, so it should work in both places)

Change-Id: I9803a127dd0be6975504e4fe55ccb2acea13d9af
Reviewed-on: https://gerrit.instructure.com/37622
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Derek DeVries <ddevries@instructure.com>
QA-Review: Caleb Guanzon <cguanzon@instructure.com>
Product-Review: Ahmad Amireh <ahmad@instructure.com>