submission icons for gb2

fixes CNVS-7916
fixes CNVS-6924

test plan:
 - submission images should be icons
 - keyboard shortcut 't' should show what
   submission type the icon refers to

Change-Id: Ie853d737743dbedadfb64fdceadb018186f44552
Reviewed-on: https://gerrit.instructure.com/33089
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>