Quiz Stats - Refactor/get rid of QuestionAnalyzer

  - removed the question analyzer
  - each answer analyzer is now expected to calculate its own
    "responses" field as opposed to doing it in a generic manner
  - made the Essay analyzer generate its "responses" field
  - a self-documenting DSL for defining analyzer output metrics

Closes CNVS-13157

TEST PLAN
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It's all code movement/refactor so nothing new to test. The plan is to
verify the Essay question stats are unaffected (which is basically all
the gem supports at this point:)

  - create a quiz with an essay question
  - take the quiz by a few students
  - grade a submission
  - retrieve the stats via the API:
    GET /api/v1/courses/:course_id/quizzes/:quiz_id/statistics
    - verify the essay stats in the API are still functional, the
      "responses" field in particular

Change-Id: I42b9552c60ccb56f7c6912fed7cc1173da71852d
Reviewed-on: https://gerrit.instructure.com/35096
Reviewed-by: Jason Madsen <jmadsen@instructure.com>
QA-Review: Caleb Guanzon <cguanzon@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Ahmad Amireh <ahmad@instructure.com>