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/shared/features/notifiable_in_app.rb

@@ -1,5 +1,4 @@
 shared_examples "notifiable in-app" do |factory_name|
-
   let(:author) { create(:user, :verified) }
   let!(:notifiable) { create(factory_name, author: author) }
 
@@ -134,7 +133,5 @@ shared_examples "notifiable in-app" do |factory_name|
       find(".icon-no-notification").click
       expect(page).to have_css ".notification", count: 0
     end
-
   end
-
 end