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/pages_spec.rb

@@ -1,14 +1,12 @@
 require "rails_helper"
 
 describe "Admin custom pages" do
-
   before do
     admin = create(:administrator)
     login_as(admin.user)
   end
 
   context "Index" do
-
     scenario "lists all created custom pages" do
       custom_page = create(:site_customization_page)
       visit admin_site_customization_pages_path
@@ -33,7 +31,6 @@ describe "Admin custom pages" do
         expect(page).to have_content slug
       end
     end
-
   end
 
   context "Create" do