fix user order_by_sortable_name when used with uniq

Change-Id: I49ef9e0cc05fe233ae4b984d58cfb06f683363ca
Reviewed-on: https://gerrit.instructure.com/28770
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Cody Cutrer <cody@instructure.com>
QA-Review: James Williams  <jamesw@instructure.com>
Product-Review: James Williams  <jamesw@instructure.com>