Quiz Stats - Multiple Choice & True/False

Changes:

  - no longer exposing user_ids
  - no longer exposing response_values
  - new metric: "responses" at the root-level

Closes CNVS-13385

TEST PLAN
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  - create a quiz with those question types
  - take the quiz by a number of students, choose different answers
  - get the stats from the API:
    [GET] /api/v1/courses/:course_id/quizzes/:quiz_id/statistics
    - verify you get the documented metrics and they have the proper
      values
  - verify the relevant sections in ember quiz stats is still functional

Change-Id: I470912060042405629812585ef6667693b1302a1
Reviewed-on: https://gerrit.instructure.com/35734
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Caleb Guanzon <cguanzon@instructure.com>
Reviewed-by: Derek DeVries <ddevries@instructure.com>
Product-Review: Derek DeVries <ddevries@instructure.com>