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

config/routes/admin.rb

@@ -66,7 +66,6 @@ namespace :admin do
     end
 
     resources :budget_phases, only: [:edit, :update]
-
   end
 
   resources :milestone_statuses, only: [:index, :new, :create, :update, :edit, :destroy]