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/models/concerns/sluggable.rb

@@ -1,7 +1,6 @@
 require "spec_helper"
 
 shared_examples_for "sluggable" do |updatable_slug_trait:|
-
   describe "generate_slug" do
     let(:factory_name) { described_class.name.parameterize(separator: "_").to_sym }
     let(:sluggable) { create(factory_name, name: "Marló Brañido Carlo") }