DA - calendar events, syllabus, assignments_needing_submitting

fixes CNVS-14100
fixes CNVS-14099
refs CNVS-14443

test plan:
  - set up differentiated assignments
  - as the teacher, all assignment events as visible as
    usual on calendar and syllabus
  - as a student, assignments without visibility and assignments
    with visibility but no applicable override (aka - the student
    only can see the assignment because of a grade) dont show up
    on the calendar or syllabus
      - also, their json doesnt come in at all when the calendar
        calls the the CalendarEvent API (ping me if you want to
        know how to check thos)
  - assignments_needing_submitting works as expected

Change-Id: I62c0b77d562808636a9a5a146eae16e25a55ffe8
Reviewed-on: https://gerrit.instructure.com/38172
Reviewed-by: Mike Nomitch <mnomitch@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Amber Taniuchi <amber@instructure.com>
Reviewed-by: Simon Williams <simon@instructure.com>
Product-Review: Hilary Scharton <hilary@instructure.com>