extract student to teacher mapping

fixes CNVS-12370

test plan:
course alerts should still work

Change-Id: I877f27506e1c8f5c89de77c266018824b7f5045e
Reviewed-on: https://gerrit.instructure.com/33252
Reviewed-by: Cody Cutrer <cody@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Jeremy Putnam <jeremyp@instructure.com>
Product-Review: Simon Williams <simon@instructure.com>