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/helpers/users_helper_spec.rb

@@ -1,7 +1,6 @@
 require "rails_helper"
 
 describe UsersHelper do
-
   describe "#humanize_document_type" do
     it "returns a humanized document type" do
       expect(humanize_document_type("1")).to eq "DNI"
@@ -55,5 +54,4 @@ describe UsersHelper do
                                                        '</del> <span class="small">(This debate has been deleted)</span>')
     end
   end
-
 end