include custom students in course group category lists

test plan:
 - create a custom student role
 - enroll one or more students in a course with this role
 - also enroll some normal students
 - go to the groups page, and create a group set
 - both normal students and custom students should show up
   in the "unassigned" list and be assignable to groups
 - if you divide students into groups automatically, both
   custom and normal students should be assigned

fixes CNVS-10434

Change-Id: Ib609af34063e10af605c8d8e11cde50d1fee967a
Reviewed-on: https://gerrit.instructure.com/29667
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Bracken Mosbacker <bracken@instructure.com>
Product-Review: Bracken Mosbacker <bracken@instructure.com>
QA-Review: Nathan Rogowski <nathan@instructure.com>