support for reordering custom gradebook columns

refs CNVS-5856

Test plan:
  * create some custom gradebook columns
  * drag them into a different order in gradebook2
  * after reloading, the order of custom columns should be preserved

Change-Id: Ib3ef172b01baabe8480b12846775fde965880215
Reviewed-on: https://gerrit.instructure.com/28155
Reviewed-by: Mike Nomitch <mnomitch@instructure.com>
Product-Review: Cameron Matheson <cameron@instructure.com>
QA-Review: Caleb Guanzon <cguanzon@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Simon Williams <simon@instructure.com>