add section_id to submissions API when grouped

Canvas submissions API can group submissions by user, but does not
currently provide the section that the user is in. We need a
section_id in order for SIS integrations with powerschool to work
since the powerschool data model is different wrt how assignments
are stored (assignments are stored in sections in PS, but in
courses in Canvas).

Test Plan:
- Query submissions API via students, e.g.

curl -H 'Authorization: Bearer [yourtoken]' -XGET
     http://localhost:3000/api/v1/courses/1/students/submissions
     -F 'student_ids=all' -F 'grouped=false'

- section_id should now be included alongside user_id

Fixes SIS-301

Change-Id: I1c0ad639a088fa9e0b8015a25adf48fec64a9e0f
Reviewed-on: https://gerrit.instructure.com/35603
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Eric Adams <eadams@instructure.com>
Reviewed-by: Simon Williams <simon@instructure.com>
Reviewed-by: Eric Adams <eadams@instructure.com>
QA-Review: Jeremy Putnam <jeremyp@instructure.com>