Change the restrictions for using self.

Based on in office conversation the use of self to reference the class
has the potential benefit of handling renames in the event that the
class name needs to change. Additionally it makes for easier extraction
into a new class.
Still implicit, but my example would have been for class-level statements.

```ruby
class User
  delegate :model_name, to: self # implicit
  def self.model_name; end # still implicit
  def model_name
    self.class.model_name # also implicit
  end
  def model_name
    User.model_name # explicit
  end
end