Show warning message when trying to create SU with existing name

Disallow creating a new supervised user if there is already existing SU user with the same name locally. If there are multiple supervised users with the same name, then disallow creating when all of them are imported locally. If at least one of them is not, show the import dialog.

BUG=557445

Review URL: https://codereview.chromium.org/1506353007

Cr-Commit-Position: refs/heads/master@{#366364}
4 files changed