fix add assignment/group buttons on assignment index page

fixes CNVS-15794

test plan:
- go to the assignment index page as a teacher
- you should have buttons at the top to add the assignment/group

Change-Id: I27c305065698195f0071fdfdcd4e985551cdb63b
Reviewed-on: https://gerrit.instructure.com/41727
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Mike Nomitch <mnomitch@instructure.com>
QA-Review: Amber Taniuchi <amber@instructure.com>
Product-Review: Simon Williams <simon@instructure.com>