designers can see unpublished assignments

fixes CNVS-15059

test plan:
  - enroll a designer in a course.
  - create two assignments, but only publish one.
  - masquerade as the designer
  - navigate to the assignments index page
    - both assignments should be showing

Change-Id: I361bc3bda4bd85539f25e94b188506c8d4ca0f54
Reviewed-on: https://gerrit.instructure.com/40125
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Matt Berns <mberns@instructure.com>
Reviewed-by: Simon Williams <simon@instructure.com>
QA-Review: Caleb Guanzon <cguanzon@instructure.com>
Product-Review: Simon Williams <simon@instructure.com>