show download submissions iff submissions exist

fixes CNVS-11691

test plan:
  - go to GB2
    - assignment with turned in submissions
      - should show Download Submissions in dropdown (even if none are graded)
    - assignments with submissions enabled but w/out submissions
      - shouldn't show Download Submissions in dropdown
    - assignments without submissions enabled
      - shouldn't show Download Submissions in dropdown
  - a large course with many assignments should load normally on GB2
  - srgb shouldn't blow up (testable changes will come with cnvs-11549)

Change-Id: Ia2619828640bfaa04d73192e881a6d2ce0e97f3e
Reviewed-on: https://gerrit.instructure.com/31718
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Caleb Guanzon <cguanzon@instructure.com>
Reviewed-by: Cameron Sutter <csutter@instructure.com>
Product-Review: Mike Nomitch <mnomitch@instructure.com>