Reinstate `is_admin` for users api `/users/ID` route

Description

The is_admin flag was removed as part of https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/10846

It has been reinstated for the /user route (https://gitlab.com/gitlab-org/gitlab-ce/issues/34325), for the /users route (https://gitlab.com/gitlab-org/gitlab-ce/issues/33260).

Why it hasn't yet been reinstated for the /users/ID route if current user is admin?

Proposal

Either fully reinstate the flag allowing admins to see the is_admin flag in /users/ID.

Links / references

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/10846

https://gitlab.com/gitlab-org/gitlab-ce/issues/33260

https://gitlab.com/gitlab-org/gitlab-ce/issues/34325

Overview

If I as admin can see is_admin by doing GET /user?sudo=ID so why I can't do this directly through GET /users/ID?