Quiz Stats - don't count teacher previews

Exclude preview quiz submission from student analysis (item analysis was
already accounting for this) based on the "was_preview" field.

Closes CNVS-8203

TEST PLAN
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  - create a quiz
  - take it as a preview by the teacher
    - request the stats from the API (or visit ember quiz stats)
    - verify that your attempt does not show up
  - take it by a student
    - verify everything is ok
  - now preview it again as a teacher
    - verify your attempt still doesn't add to the stats

Change-Id: I62bf1dd6c0bedb26ee55d029ac0f015b2ad48e91
Reviewed-on: https://gerrit.instructure.com/35954
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Derek DeVries <ddevries@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>
Product-Review: Derek DeVries <ddevries@instructure.com>