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/investments_controller.rb

@@ -1,6 +1,5 @@
 module Budgets
   class InvestmentsController < ApplicationController
-
     include FeatureFlags
     include CommentableActions
     include FlagActions
@@ -187,7 +186,5 @@ module Budgets
       def load_map
         @map_location = MapLocation.load_from_heading(@heading)
       end
-
   end
-
 end