DA - remove N+1 query in GB api calls

fixes CNVS-15819

test plan:
 * with DA on
 - make DA assignments
 > GB should load assignments without errors
 * using the api
 - get /api/v1/courses/:id/assignments and make sure to include
   "include_visibility"
 > it should include the user ids that have visibility as an array
 > it should not throw an error

Change-Id: I978e07188c723626c683ccd07d5c6e9854d94d24
Reviewed-on: https://gerrit.instructure.com/41849
Reviewed-by: Mike Nomitch <mnomitch@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Amber Taniuchi <amber@instructure.com>
Product-Review: Cameron Sutter <csutter@instructure.com>