make never_drop ids string for grade summary ENV

fixes CNVS-10864

test plan
=========

- add drop rules to never drop one or more assignments
- verify that the totals in Gradebook, GB2, and the student grades
  summary page match

Change-Id: I1ac4a31a03b664ac6d3e92bc2cd761c69d2056b5
Reviewed-on: https://gerrit.instructure.com/29619
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Caleb Guanzon <cguanzon@instructure.com>
Reviewed-by: Cameron Sutter <csutter@instructure.com>
Product-Review: Simon Williams <simon@instructure.com>