speed up assignment index loading

fixes CNVS-16556

test plan:
  - assignment index loads fine as teacher & student
  - as does the gradebook
  - as do calls to the assignment group api

Change-Id: Iff2fe084c15500f1dd232606f9182df328c76988
Reviewed-on: https://gerrit.instructure.com/43511
Reviewed-by: Cameron Sutter <csutter@instructure.com>
Reviewed-by: Matt Berns <mberns@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Sean Lewis <slewis@instructure.com>
Product-Review: Simon Williams <simon@instructure.com>