remove N+1 from student grades page
remove N+1 from student grades page

Test Plan
- Create a lot of assignments.
- Load the grades page as a student and verify in the server output that
there are not a lot of `SELECT * FROM post_policies...`

Change-Id: Ic606a50c081f1ff3dbc6158cb26edc1d6f85acec
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/232100
Reviewed-by: Adrian Packel <9c6a39807293376f29cba17542b4cb1b663ecc4a@instructure.com>
Reviewed-by: Spencer Olson <7cc0890d6f1f70810a023e7173eca5724d6c8e49@instructure.com>
Tested-by: Service Cloud Jenkins <9144042a601061f88f1e1d7a1753ea3e2972119d@instructure.com>
QA-Review: Keith Garner <a1a80143a840c140f31c3495ebbd9ec906e09fed@instructure.com>
Product-Review: Keith Garner <a1a80143a840c140f31c3495ebbd9ec906e09fed@instructure.com>

sfu/canvas-lms,sfu/canvas-lms,djbender/canvas-lms,djbender/canvas-lms,djbender/canvas-lms,sfu/canvas-lms,djbender/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms