don't show the 'everyone' assignment date if all sections have overrides

fixes CNVS-13002

test plan:
  - as a teacher
    - create a quiz with assignment overrides for every section
    - enable fabulous quizzes
    - visit the quiz
    - make sure that it doesn't show an extra date for "Everyone"

  - as a teacher
    - check the courses/{id}/quizzes/{id} jsonapi end point
    - there should be a new 'section_count' attribute

  - as a student
    - check the courses/{id}/quizzes/{id} jsonapi end point
    - there should *NOT* be a 'section_count' attribute

Change-Id: I1cf33be890c76df07d73cfa278dcbc550abf1090
Reviewed-on: https://gerrit.instructure.com/34791
Reviewed-by: Ahmad Amireh <ahmad@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Caleb Guanzon <cguanzon@instructure.com>
Product-Review: Derek DeVries <ddevries@instructure.com>