hide scores from students in attempts table for muted submissions

fixes CNVS-8208

test plan:
  - create a new quiz as a teacher
  - mute the quiz in gradebook
  - have a student take the quiz
  - the student score in the attempts table should say "Not yet graded"

Change-Id: Ief963ee17fdf42117ebd0a47bf206493e82dcab1
Reviewed-on: https://gerrit.instructure.com/24540
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Josh Simpson <jsimpson@instructure.com>
Product-Review: Derek DeVries <ddevries@instructure.com>
QA-Review: Myller de Araujo <myller@instructure.com>
treesame commit of origin/stable/2013-09-14

Treesame-Commit-Id: 3b6790d8cf364dd73b6873987b743125d6d38109