Support enums and scopes on super class in STI models

[tk0miya]

ActiveRecord supports STI (Single Table Inheritance) via class inheritation.
If the parent class of the STI model has enum definitions, scopes, and so on, the child classes can use them also.

Example:

class User < ActiveRecord::Base
  scope :active, -> { where(is_active: 1) }
end

class AdminUser < User
end

AdminUser.active  # scopes defined in the parent class are available in the child classes.