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/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