add belongs to root_account on AssignmentOverride
add belongs to root_account on AssignmentOverride

the ids were added long ago, but not the association
this makes messages resolve the root_account to give better urls

test plan
 - existing specs should pass

refs VICE-2308
flag=none

Change-Id: I52c3ff96079d28097f85e2e85210fc4007e2146c
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/279292
Tested-by: Service Cloud Jenkins <9144042a601061f88f1e1d7a1753ea3e2972119d@instructure.com>
Reviewed-by: Matthew Lemon <7bd6faf60934a850234395b3b97cb1fc9250fa11@instructure.com>
QA-Review: Matthew Lemon <7bd6faf60934a850234395b3b97cb1fc9250fa11@instructure.com>
Product-Review: Matthew Lemon <7bd6faf60934a850234395b3b97cb1fc9250fa11@instructure.com>

sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms