assignment migration for only_visible_to_overrides flag

also updates API to send/receive this flag

fixes CNVS-9762
fixes CNVS-9764

test plan:
 - turn on differentiated assignments flag in account & course
 - request an assignment via api
   - it should include this flag
 - update this flag on an assignment via api
   - it should work

Change-Id: Ibc01a7bc3effef8605c622f3e644ddc4e10effa3
Reviewed-on: https://gerrit.instructure.com/29526
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Simon Williams <simon@instructure.com>
QA-Review: Amber Taniuchi <amber@instructure.com>
Product-Review: Simon Williams <simon@instructure.com>