use ids instead of names for roles in permissions UI

test plan:
* regression test the account "Permissions" page
* also confirm that deleting a role with a "/" character
 in the name works

fixes #CNVS-6278

Change-Id: I521bf25dc9e3b335127a11c61cc204e3c43a7b9d
Reviewed-on: https://gerrit.instructure.com/43738
Reviewed-by: Jeremy Stanley <jeremy@instructure.com>
Product-Review: Jeremy Stanley <jeremy@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Anna Koalenz <akoalenz@instructure.com>