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/features/admin/valuators_spec.rb

@@ -1,7 +1,6 @@
 require "rails_helper"
 
 describe "Admin valuators" do
-
   let(:admin) { create(:administrator) }
   let!(:user) { create(:user, username: "Jose Luis Balbin") }
   let!(:valuator) { create(:valuator) }
@@ -110,5 +109,4 @@ describe "Admin valuators" do
       expect(page).not_to have_content(valuator1.email)
     end
   end
-
 end