Allow users to opt-out from the ActionController extensions

My take on https://github.com/rails-api/active_model_serializers/pull/589

That way users can set `ActionController::Serialization.enabled = false` in an initializer or environment file.

The `after_initialize` hook is slightly unusual, but as far as I can tell it doesn't have any downside.

@steveklabnik is that solution acceptable?

cc @tjoyal, @christianblais, @arthurnn