Reinstate `is_admin` for users api `/user` 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 /users route (https://gitlab.com/gitlab-org/gitlab-ce/issues/33260) allowing an admin to query other users (or themselves) for the is_admin flag.

It hasn't yet been reinstated for the /user route - which is used in packages such as the socialite package.

Proposal

Either fully reinstate the flag allowing admins and non-admins to see the is_admin flag in /user, or reinstate it just for admins - the absence of the flag will pretty much be the same as a false value.

Links / references

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

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

Overview

We used the is_admin flag within an internal tool (which used the GitLab socialite package), which we only wanted GitLab admins to access. We've had to temporarily convert to a user id whitelist until this is reinstated.