Quiz Stats [Backend] - Gem & Essays

Refactoring the generation of quiz question statistics into its own gem.
This patch adds support for Essay question statistics.

Closes CNVS-12725

TEST PLAN
---- ----

  - create a quiz with an essay question
  - perform an API request to retrieve the quiz statistics
  - ensure that the following metrics are generated and correct:
    - "graded": number of students whose answers have been graded by the
      teacher
    - "full_credit": number of students who received a full score
    - "point_distribution": a list of scores and the number of students
      who received them (so if 2 students got graded for 3 points, it
      should have a key of 2 and a value of 3), un-graded submissions
      should be keyed under null
    - "responses": number of students who answered the question
      (wrote anything)
    - "user_ids": IDs of those students
  - documentation for QuizStatistics -> Essay should be updated with the
    new stats

> Other things to test

  - verify that the old statistics page still renders:
    /courses/:course_id/quizzes/:quiz_id/statistics
  - verify that you can still generate both student and item analysis
    CSV reports

API endpoint for quiz stats:

  /api/v1/courses/:courseid/quizzes/:quiz_id/statistics [GET]

Change-Id: Ib15434ff4cef89ac211c1f4602d1ee609ef48ec4
Reviewed-on: https://gerrit.instructure.com/33990
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Caleb Guanzon <cguanzon@instructure.com>
Reviewed-by: Derek DeVries <ddevries@instructure.com>
Product-Review: Ahmad Amireh <ahmad@instructure.com>