prevent ungrouped users from submitting to group assignments

fixes CNVS-15524

Test plan:
  * Make a group set with no groups
  * Make an assignment with that group set
  * No students should be able to submit to the assignment
  * Make some groups.  Add (some) students to one of the groups
  * Students in that group should be able to submit, others still can't
    submit
  * the speedgrader/downloading submissions should work

Change-Id: Ic4357b1fc28dfe6408de3b98fdea279757995a84
Reviewed-on: https://gerrit.instructure.com/41297
Reviewed-by: Simon Williams <simon@instructure.com>
QA-Review: Sean Lewis <slewis@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Cameron Matheson <cameron@instructure.com>