handle cross-domain account association check

fixes CNVS-13042

test-plan:
 - have multiple shards
 - have a course on shard A
 - have a two users on shard B
 - enroll one user in course as teacher
 - enroll other user in course as student
 - make a submission as the student, then grade that submission as the
   teacher
 - have an admin user on shard A view the grade change audit admin tool
 - search for events for the student user; should find them
 - repeat for the teacher (grader) user; should find them

Change-Id: Ia35aef892dae0825f721e37aafa0baed4a038f5a
Reviewed-on: https://gerrit.instructure.com/36570
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Cody Cutrer <cody@instructure.com>
QA-Review: August Thornton <august@instructure.com>
Product-Review: Jacob Fugal <jacob@instructure.com>