Separate UI and API checks for user impersonation

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/166639



Merged-by: Mireya Andres <mandres@gitlab.com>
Approved-by: Drew Blessing <drew@gitlab.com>
Approved-by: Vanessa Otto <votto@gitlab.com>
Approved-by: Mireya Andres <mandres@gitlab.com>
Reviewed-by: Drew Blessing <drew@gitlab.com>
Co-authored-by: habdul-razak <habdul-razak@gitlab.com>
Co-authored-by: Hakeem Abdul-Razak <hakeem@hakeems-mbp.lan>