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/users/registrations_controller_spec.rb

@@ -1,9 +1,7 @@
 require "rails_helper"
 
 describe Users::RegistrationsController do
-
   describe "POST check_username" do
-
     before do
       request.env["devise.mapping"] = Devise.mappings[:user]
     end
@@ -28,7 +26,5 @@ describe Users::RegistrationsController do
         expect(data[:message]).to eq I18n.t("devise_views.users.registrations.new.username_is_not_available")
       end
     end
-
   end
-
 end