fixes#9601
Test plan:
* fill out the 'Add Course Users' section in course settings.
* try to add existing users and new users.
* make sure everything looks OK.
Change-Id: I688d3a583fda663c737e78dfa33973e1b3a7497d
Reviewed-on: https://gerrit.instructure.com/12471
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Zach Pendleton <zachp@instructure.com>