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

spec/controllers/related_contents_controller_spec.rb

@@ -1,7 +1,6 @@
 require "rails_helper"
 
 describe RelatedContentsController do
-
   describe "#score" do
     it "raises an error if related content does not exist" do
       controller.params[:id] = 0
@@ -11,5 +10,4 @@ describe RelatedContentsController do
       end.to raise_error ActiveRecord::RecordNotFound
     end
   end
-
 end