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/nested_imageable.rb

@@ -17,7 +17,6 @@ shared_examples "nested imageable" do |imageable_factory_name, path, imageable_p
   end
 
   describe "at #{path}" do
-
     scenario "Should show new image link when imageable has not an associated image defined" do
       login_as user
       visit send(path, arguments)
@@ -250,9 +249,7 @@ shared_examples "nested imageable" do |imageable_factory_name, path, imageable_p
       end
 
     end
-
   end
-
 end
 
 def imageable_redirected_to_resource_show_or_navigate_to