Internationalize all model error messages

[david-yz-liu]

While built-in model validations are internationalized, several of our custom model validations use English string literals, e.g.

Markus/app/models/admin_role.rb

Lines 4 to 9 in a43f781

	def associated_user_is_an_admin
	unless self.user.nil? || self.user.admin_user?
	errors.add(:base, 'only admin users can be assigned the admin role')
	end
	end
	end

We should internationalize all of these strings. We can follow the same structure as the built-in error message scopes, e.g.

en:
  activerecord
    errors:
      models:
        admin_role:
          associated_user_is_an_admin: "Only admin users..."

and then in the model,

  def associated_user_is_an_admin
    unless self.user.nil? || self.user.admin_user?
      errors.add(:base, :associated_user_is_an_admin)
    end
  end

See also Errors#add.