update course roster user addition to use role_ids

test plan:
* should be able to add people to custom and built-in
roles through the course "People" page as before

closes #CNVS-17496

Change-Id: I25967ad06a2dde5a327963cf47347cf9cc889093
Reviewed-on: https://gerrit.instructure.com/45769
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Dan Minkevitch <dan@instructure.com>
QA-Review: Jahnavi Yetukuri <jyetukuri@instructure.com>
Product-Review: James Williams  <jamesw@instructure.com>