use an inverse association instead of an include for Course#enrollments#course

Change-Id: Ief4e0423b3af846d235e35d566ea7cd6e32b0ab0
Reviewed-on: https://gerrit.instructure.com/40978
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Brian Palmer <brianp@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>