show groupless submissions in speedgrader

fixes CNVS-15753

Test plan:
  * make a group assignment with a group category that has no groups
  * submit some homework
  * the submissions should display (and be gradeable) in speedgrader
  * add some groups (with people in them)
  * submit more homework for the people with groups
  * the submissions for people in groups and people not in groups
    should all display
  * downloading submissions should work as well

Change-Id: I7e6081cec61936c4f833394e995963698f4b57b7
Reviewed-on: https://gerrit.instructure.com/41596
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Matt Berns <mberns@instructure.com>
Reviewed-by: Mike Nomitch <mnomitch@instructure.com>
QA-Review: Sean Lewis <slewis@instructure.com>
Product-Review: Simon Williams <simon@instructure.com>