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/signature_sheets_helper_spec.rb

@@ -1,18 +1,14 @@
 require "rails_helper"
 
 describe SignatureSheetsHelper do
-
   describe "#required_fields_to_verify_text_help by default" do
-
     it "returns text help by default" do
       text = "Write the numbers separated by semicolons (;)"
       expect(required_fields_to_verify_text_help).to eq(text)
     end
-
   end
 
   describe "#required_fields_to_verify_text_help with remote_census active" do
-
     before do
       Setting["feature.remote_census"] = true
     end
@@ -70,7 +66,5 @@ describe SignatureSheetsHelper do
       Setting["remote_census.request.postal_code"] = nil
       Setting["remote_census.request.postal_code"] = nil
     end
-
   end
-
 end