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/customization_engine_spec.rb

@@ -4,7 +4,6 @@ require "rails_helper"
 # TODO test models, controllers, etc...
 
 describe "Customization Engine" do
-
   let(:test_key)      { I18n.t("account.show.change_credentials_link") }
   let!(:default_path) { I18n.load_path }
 
@@ -37,5 +36,4 @@ describe "Customization Engine" do
     I18n.load_path += path
     I18n.reload!
   end
-
 end