Add integration_id to submissions API

Added integration_id to the user (pseudonym) as well as the
assignments when querying for submissions.

Test Plan:
- query for submissions from a student with an sis_id and
  an integration_id, and for an assignment with an integration_id
- use grouped=false
- results should have integration_id for students, and
  integration_id for assignments
- e.g. curl -H 'Authorization: Bearer [your API key]' \
  -XGET http://localhost:3000/api/v1/courses/1/students/submissions \
  -F 'include[]=assignment' -F 'student_ids=all' -F 'grouped=false'

Fixes SIS-252

Change-Id: I0339fa19bc50dfea3719598924a90746b62f7ebe
Reviewed-on: https://gerrit.instructure.com/34456
Reviewed-by: Eric Adams <eadams@instructure.com>
Product-Review: Eric Adams <eadams@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Jeremy Putnam <jeremyp@instructure.com>