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/budgets/stats_controller.rb

@@ -1,6 +1,5 @@
 module Budgets
   class StatsController < ApplicationController
-
     before_action :load_budget
     load_and_authorize_resource :budget
 
@@ -15,6 +14,5 @@ module Budgets
       def load_budget
         @budget = Budget.find_by_slug_or_id! params[:budget_id]
       end
-
   end
 end