Add and apply rubocop rules for empty lines

We were very inconsistent regarding these rules.

Personally I prefer no empty lines around blocks, clases, etc... as
recommended by the Ruby style guide [1], and they're the default values
in rubocop, so those are the settings I'm applying.

The exception is the `private` access modifier, since we were leaving
empty lines around it most of the time. That's the default rubocop rule
as well. Personally I don't have a strong preference about this one.


[1] https://rubystyle.guide/#empty-lines-around-bodies

app/models/concerns/graphqlable.rb

@@ -2,7 +2,6 @@ module Graphqlable
   extend ActiveSupport::Concern
 
   class_methods do
-
     def graphql_field_name
       name.gsub("::", "_").underscore.to_sym
     end
@@ -26,11 +25,9 @@ module Graphqlable
     def graphql_type_description
       model_name.human.to_s
     end
-
   end
 
   def public_created_at
     created_at.change(min: 0)
   end
-
 end