Quiz Stats - Maintain ERB compatibility

This patch makes it so that if you want to get the new stats from the
back-end, you'll have to explicitly pass a "legacy=false" parameter to
the report generator routine. Old code will use the old output, new code
will use the CanvasQuizStatistics gem for stats.

Closes CNVS-13198

TEST PLAN
---- ----

  - create a quiz with a bunch of questions and take it by multiple
    students
  - visit the ERB stats page at
    /courses/:course_id/quizzes/:quiz_id/statistics
    - verify that the page renders and looks just like how it was before
      the work on its ember counterpart
  - visit the ember stats page at
    /courses/:course_id/quizzes/fabulous_quizzes#:quiz_id/statistics
    - verify the ember page is still functional

Change-Id: I0a9e8d69eacc64a8727f238fc0c2e2acd44c0451
Reviewed-on: https://gerrit.instructure.com/35167
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>