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/site_customization/information_texts_spec.rb

@@ -1,7 +1,6 @@
 require "rails_helper"
 
 describe "Admin custom information texts" do
-
   before do
     admin = create(:administrator)
     login_as(admin.user)
@@ -57,7 +56,6 @@ describe "Admin custom information texts" do
   end
 
   context "Globalization" do
-
     scenario "Add a translation", :js do
       key = "debates.index.section_footer.title"
 
@@ -130,5 +128,4 @@ describe "Admin custom information texts" do
       expect(debate_title.value_en).to eq("Custom debate title")
     end
   end
-
 end