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/admin_helper.rb

@@ -1,5 +1,4 @@
 module AdminHelper
-
   def side_menu
     if namespace == "moderation/budgets"
       render "/moderation/menu"
@@ -114,5 +113,4 @@ module AdminHelper
     def namespace
       controller.class.name.downcase.split("::").first
     end
-
 end