replace usages of Quiz#find_or_create_submission with SubmissionManager#find_or_create_submission

fixes: CNVS-12598

  test plan:
    - create a quiz as an instructor
    - as a student
    - take quiz
    - exercise all paths by which a quiz submission is generated
    - verify that quiz submissions continue to work as before

Change-Id: I9fd71679e6c9a45686861609ff481f5f4250cca1
Reviewed-on: https://gerrit.instructure.com/33735
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Derek DeVries <ddevries@instructure.com>
QA-Review: Caleb Guanzon <cguanzon@instructure.com>
Product-Review: Jason Madsen <jmadsen@instructure.com>