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/features/admin/administrators_spec.rb

@@ -40,7 +40,6 @@ describe "Admin administrators" do
   end
 
   scenario "Delete Administrator when its the current user" do
-
     within "#administrator_#{admin.id}" do
       click_on "Delete"
     end
@@ -115,5 +114,4 @@ describe "Admin administrators" do
       expect(page).to have_content("Admin Alias")
     end
   end
-
 end