MGP - filter submissions

fixes CNVS-17104

test plan:
 * turn on MGP feature flag
 * using the submissions api
   /api/v1/courses/1/students/submissions
 * with the same parameters as gb uses
   - grouped=1
   - list of student ids (student_ids[]=<id>&student_ids[]=<id>)
 - add the grading_period_id parameter with the appropriate id
   - &grading_period_id=<id>
 > the response should only include submissions
   from assignments in that grading period

Change-Id: Ic1ff47564bf16cd1d158ee6a3307ed76a96915c9
Reviewed-on: https://gerrit.instructure.com/45404
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Josh Simpson <jsimpson@instructure.com>
QA-Review: Amber Taniuchi <amber@instructure.com>
Product-Review: Cameron Sutter <csutter@instructure.com>