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/controllers/application_controller_spec.rb

@@ -1,9 +1,7 @@
 require "rails_helper"
 
 describe ApplicationController do
-
   describe "#current_budget" do
-
     it "returns the last budget that is not in draft phase" do
       create(:budget, :finished,  created_at: 2.years.ago, name: "Old")
       create(:budget, :accepting, created_at: 1.year.ago,  name: "Previous")
@@ -13,7 +11,5 @@ describe ApplicationController do
       budget = subject.instance_eval { current_budget }
       expect(budget.name).to eq("Current")
     end
-
   end
-
 end