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

app/controllers/admin/site_customization/base_controller.rb

@@ -6,5 +6,4 @@ class Admin::SiteCustomization::BaseController < Admin::BaseController
     def namespace
       "admin"
     end
-
 end

app/controllers/admin/site_customization/cards_controller.rb

@@ -5,5 +5,4 @@ class Admin::SiteCustomization::CardsController < Admin::SiteCustomization::Base
     @page = ::SiteCustomization::Page.find(params[:page_id])
     @cards = @page.cards
   end
-
 end

app/controllers/admin/site_customization/documents_controller.rb

@@ -1,5 +1,4 @@
 class Admin::SiteCustomization::DocumentsController < Admin::SiteCustomization::BaseController
-
   def index
     @documents = Document.admin.page(params[:page])
   end

app/controllers/admin/site_customization/information_texts_controller.rb

@@ -23,7 +23,6 @@ class Admin::SiteCustomization::InformationTextsController < Admin::SiteCustomiz
           end
         end
       end
-
     end
 
     redirect_to admin_site_customization_information_texts_path,