Quizzes - Lock down results after first view

Adds an option to Quiz that prevents the students from seeing their
results more than once after their submission.

Closes CNVS-13877

CHANGES
-------

  - New option in quiz settings:
    "Let Student See Their Responses" => "Only Once After Each Attempt"
  - Teacher can always see the student's results
  - Moderate dialog allows teacher to manually reset the result-locking
    flag, allowing the student one more glimpse
  - The flag is reset whenever a new attempt is taken

TEST PLAN
---- ----

  - create a quiz
  - toggle the new option: "Let Student See Their Responses" then "Only
    Once After Each Attempt"

Turn off Ember/quiz stats:

  - take the quiz by a student, turn it in:
    - verify that you see your results
    - reload the page
      - verify that you no longer see the results
  - take the quiz again:
    - verify you see the results once, then not
  - go to the moderate page as the teacher
    - open the moderate dialog for that student
      - check the new option "Let the student see the results one more
        time"
      - click Save
        - reload the page as the student, verify that you see the
          results

Turn on Ember/quiz stats:

  - repeat the process

Other things to test:

  - verify the new field "has_seen_results" is visible in the
    submissions API and is documented
  - verify the new field "one_time_results" is visible in the quizzes
    API and is documented properly
  - verify that the new option has no effect if the Quiz option is not
    set
  - verify that you can always see the student's results as a teacher
    (the setting should not affect teachers)

Change-Id: I6eb4909f777e038acf6afc9ef2823352f5ea78be
Reviewed-on: https://gerrit.instructure.com/37544
QA-Review: Caleb Guanzon <cguanzon@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Derek DeVries <ddevries@instructure.com>
Product-Review: Ahmad Amireh <ahmad@instructure.com>