use scopes instead of associations with joins

associations with joins were never really supported by rails

test plan:
 * canvas works

Change-Id: I62dd9b38c7340fce96351f1e3ba91acd6440aa66
Reviewed-on: https://gerrit.instructure.com/42856
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jacob Fugal <jacob@instructure.com>
QA-Review: August Thornton <august@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>