assignment show page can show multiple due dates for observers

fixes CNVS-10700

test plan:
  - as a teacher create an assignment w/ many
    different due dates
  - create an observer
    - make observer follow a student in one section
      - assignment show page displays date properly
    - make observer follow a students in many sections
      - assignment show page displays "multiple due date" and tooltip properly
    - make observer follow a student in section w/o date
      - assignmnet show page says there is no date
  - students in multiple sections see the correct date
    on the show page (not multiple dates)

Change-Id: I4e5b3872a69a59f771ca68efd4de36b410de639f
Reviewed-on: https://gerrit.instructure.com/35851
Reviewed-by: Cameron Sutter <csutter@instructure.com>
Reviewed-by: Simon Williams <simon@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Anna Koalenz <akoalenz@instructure.com>
Product-Review: Hilary Scharton <hilary@instructure.com>