add assignment group information to gradebook.csv

closes CNVS-9968

Test plan:
  Download gradebook.csv, it should include grades for each assignment
  group.  Points will be included for courses with  unweighted
  assignment groups (just like for current/final grades)

Change-Id: Ic47a4cd6cfb62726d3ec6e015b1d3ecfd71f79cc
Reviewed-on: https://gerrit.instructure.com/28579
Reviewed-by: Mike Nomitch <mnomitch@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Caleb Guanzon <cguanzon@instructure.com>
Reviewed-by: Simon Williams <simon@instructure.com>
Product-Review: Cameron Matheson <cameron@instructure.com>