Add #model_name instance method to ActiveModel::Naming

This pull request addresses #15428 and [discussion on the Rails Core mailing list](https://groups.google.com/forum/#!topic/rubyonrails-core/ThSaXw9y1F8).
Given that this is rails, I doubt a comment about my not caring for the nuances and opinions of `ActiveSupport::Concern` would be productive. I really like where you're going on this pull request, so perhaps permission to eschew any concerns about `InstanceMethods` could be given.

Either way, thanks for thinking about it!