remove ActiveAdmin::Config and replace it to ActiveAdmin::Resource

ActiveAdmin::Config class was originally created in this [PR](https://github.com/gregbell/active_admin/pull/758) but removed in this [commit](https://github.com/pcreux/active_admin/commit/c68345fc791550db2038f33ccca3057ecdaa9377).
I replaced all `ActiveAdmin::Config` to `ActiveAdmin::Resource`, since that's seems reasonable.