Add sis_user_id to submissions for multiple assignments

Test Plan:
- Build a course with an enrolled student imported by the same SisBatch
- Request submissions for multiple assignments with the grouped
  parameter included in the query
- The response should include the sis_user_id of the student

Change-Id: I9c642c839fac74d45efb47d4252c3aac3a687fb7
Fixes: SIS-660
Reviewed-on: https://gerrit.instructure.com/44319
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Nick Houle <nhoule@instructure.com>
Reviewed-by: Simon Williams <simon@instructure.com>
QA-Review: Amber Taniuchi <amber@instructure.com>
Product-Review: Tyler Pickett <tpickett+gerrit@instructure.com>