Created by: dmitry-solomadin
Related issue: https://github.com/thoughtbot/administrate/issues/951
Currently, when passing class_name option to has_one field it does not infer correct class name for the dashboard which results in exception.
This PR modifies has_one class to use associative to get the correct dashboard name.
Additionally, it fixes permitted_attributes method, which required me to modify the signature and pass options from Deferred attribute.
In scope of this I've also removed old locale keys for has_one.not_supported string.
/r @nickcharlton @tysongach