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/helpers/budget_headings_helper.rb

@@ -1,5 +1,4 @@
 module BudgetHeadingsHelper
-
   def budget_heading_select_options(budget)
     budget.headings.sort_by_name.map do |heading|
       [heading.name_scoped_by_group, heading.id]
@@ -11,5 +10,4 @@ module BudgetHeadingsHelper
     heading_path = budget_investments_path(budget, heading_id: assigned_heading&.id)
     link_to(assigned_heading.name, heading_path)
   end
-
 end