speedgrader icons

fixes CNVS-12087

test plan:
 - open speedgrader with some submissions
 - in the student selector dropdown box:
   > students without submissions should have no icon and be grayed out
   > students with ungraded submissions should have an orange dot
 - grade and ungrade a few submissions
   > graded students should have a green check mark
   > ungraded students should have an orange dot
 > students with resubmissions should be the same
   as students with ungraded submissions

Change-Id: Iaf60465c48e657c9154860561389d00c0f51eaa4
Reviewed-on: https://gerrit.instructure.com/34499
Reviewed-by: Mike Nomitch <mnomitch@instructure.com>
Product-Review: Cameron Sutter <csutter@instructure.com>
QA-Review: Trevor deHaan <tdehaan@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>