validate types on polymorphic associations

fixes ALYT-26

test plan:
	* regression test

Change-Id: Ie93e89b842e354f1f76d75f3492145b86c79a035
Reviewed-on: https://gerrit.instructure.com/34560
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jon Jensen <jon@instructure.com>
Reviewed-by: Derek DeVries <ddevries@instructure.com>
QA-Review: August Thornton <august@instructure.com>
Product-Review: Braden Anderson <banderson@instructure.com>