graphql: fix n+1 in grading period loader
graphql: fix n+1 in grading period loader

refs RECNVS-608

Test plan: specs pass

Change-Id: Ibe632b28b31d040cf757b60fa3a9d6dd4190b67c
Reviewed-on: https://gerrit.instructure.com/164589
Tested-by: Jenkins
Reviewed-by: Jonathan Featherstone <c99460343437e667b0585ea68fd7caf7bd0a02ab@instructure.com>
Product-Review: Cameron Matheson <1d572acbfa68c7c6e541c7b840d6b622e5c0dc91@instructure.com>
QA-Review: Cameron Matheson <1d572acbfa68c7c6e541c7b840d6b622e5c0dc91@instructure.com>

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