recipient search: order by sortable name

fixes CNVS-13163

test plan:
  * create a course with multiple students with full names
  * open conversations
  * open the recipient finder
  * verify that students are sorted by sortable name

Change-Id: Iadd24972c2851f3c9ccb8164ab988e271e95d27e
Reviewed-on: https://gerrit.instructure.com/35281
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Ethan Vizitei <evizitei@instructure.com>
QA-Review: Steven Shepherd <sshepherd@instructure.com>
Product-Review: Braden Anderson <banderson@instructure.com>