fix lock - unlock quiz logic in ember quiz show

closes CNVS-13490

test plan:
  - enable :quiz_stats
  - visit quiz index, select a quiz
  - verify that "Lock this Quiz now" and "Unlock this Quiz now" menu
    item works
  - consider the following cases for testing and combinations therein
    - expired lockAt date
    - future lockAt date
    - no lockAt date
    - future unlockAt date
    - passed unlockAt date
    - no unlockAt date

  - other considerations
    - when locking a quiz...
      - when no prior due date - lock and due date gets set to now
      - when due at is in the future - lock and due dates get set to now
      - expired due at date - due date stays the same, lock at gets set
        to now
    - when unlocking a quiz
      - when existing unlock date for future date - sets unlock at date
        to now
      - when lock at date exists and is already past - removes the lock
        at date

note: this commit does not address dates for individual sections of the
quiz

Change-Id: I1b0b9e1007ef15024224d82077a67b60ad95b323
Reviewed-on: https://gerrit.instructure.com/36147
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Derek DeVries <ddevries@instructure.com>
QA-Review: Caleb Guanzon <cguanzon@instructure.com>
Product-Review: Derek DeVries <ddevries@instructure.com>