Quiz Stats - Whitelist exposed question data

Changes:

  - drops unused attributes from the api output, the only things we
    expose now are question id, type, text, and position (for sorting)
  - drops logic in ember that is no longer necessary
  - stringifies IDs in the serializer output

Closes CNVS-13388

TEST PLAN
---- ----

  - BEFORE YOU CHECK THIS OUT:
    - create a quiz with all question types
    - hit the stats API and save the JSON output in some text file
  - now check this patch out:
    - use the same quiz, hit the stats api
    - verify that each question document in "question_statistics" has
      the following fields beside the metrics:
      "id", "question_type", "question_name", and "position"
    - verify that MChoice/TF questions still contain the "item analysis"
      report data (like point_biserials, etc.)
    - check out the ember quiz stats and make sure everything is OK

Note: i found a small tool that can help you see the differences in the
API output if you push the old JSON and the new one into it: try it out
maybe it will make things easier: http://tlrobinson.net/

Note 2: the docs are really out-of-date now and need to be adjusted.
There is CNVS-13387 for that.

Change-Id: I2d2e8c4dcb0e406378b50cd63f5aba14efe8c2ef
Reviewed-on: https://gerrit.instructure.com/35739
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>