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/lib/cache_spec.rb

@@ -1,7 +1,6 @@
 require "rails_helper"
 
 describe "Cache flow" do
-
   describe "Tag destroy" do
     it "invalidates Debate cache keys" do
       debate = create(:debate, tag_list: "Good, Bad")
@@ -10,5 +9,4 @@ describe "Cache flow" do
       expect { tag.destroy }.to change { debate.reload.cache_key }
     end
   end
-
 end