Quiz Stats - Account for fudge points

Fudge points were not considered because we were manually summing up the
points for each individual question. The patch makes it so that we use
the pre-calculated #score field for the submission.

Closes CNVS-13645

TEST PLAN
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  - create a quiz with 1 question with 2 points possible
  - take it by a student and answer it correctly
    - visit quiz stats, verify the numbers are now 2
  - add a -1 fudge point
    - visit quiz stats, verify that the low/high/avg scores are 1
  - manually grade the question up to 2.5 from 2
    - visit quiz stats, verify the numbers are now 1.5

Change-Id: I4341a6241b2c94b654b2ddce4439e4cbb71344e8
Reviewed-on: https://gerrit.instructure.com/38668
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Caleb Guanzon <cguanzon@instructure.com>
Reviewed-by: Derek DeVries <ddevries@instructure.com>
Product-Review: Derek DeVries <ddevries@instructure.com>