Quiz Stats - Multiple Dropdowns

Closes CNVS-13161

TEST PLAN
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  - create a quiz with multiple-dropdown questions
  - take the quiz by a number of students and type in different answers
  - make an API request to stats:
    GET /api/v1/courses/:course_id/quizzes/:quiz_id/statistics
    - verify you get the documented metrics and they have the proper
      values
  - visit the ember quizzes page and verify that:
    - the charts are still working

Change-Id: I37ef48f22c3c16170034b6dd887b40ff20da4af5
Reviewed-on: https://gerrit.instructure.com/35104
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>