APIv5: change is_admin to admin
gitlab-foss!10520 (merged) removed the User#is_admin? method from the User model.
That MR has not touched API responses (both v3 and v4).
https://gitlab.com/gitlab-org/gitlab-ce/blob/master/lib/api/entities.rb#L17
We should deprecated is_admin and rename it to admin to make our code consistent.
/cc @rspeicher