Add grade passback columns to assignments table

Test Plan:
- In console:
  - a = Assignment.new
  - a.integration_id = '12345'
  - a.post_to_sis = true
  - a.save
- it should save the two values

Fixes SIS-198

Change-Id: I9cd8e9a23db3357a8688d8cdb8b96727b8881e83
Reviewed-on: https://gerrit.instructure.com/32911
QA-Review: Jeremy Putnam <jeremyp@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Eric Adams <eadams@instructure.com>
Product-Review: Eric Adams <eadams@instructure.com>