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/verification/letter_spec.rb

@@ -1,7 +1,6 @@
 require "rails_helper"
 
 describe "Verify Letter" do
-
   scenario "Request a letter" do
     user = create(:user, residence_verified_at: Time.current,
                          confirmed_phone:       "611111111")
@@ -41,7 +40,6 @@ describe "Verify Letter" do
   end
 
   context "Code verification" do
-
     scenario "Valid verification user logged in" do
       user = create(:user, residence_verified_at: Time.current,
                            confirmed_phone:       "611111111",
@@ -129,6 +127,5 @@ describe "Verify Letter" do
       expect(page).to have_content "You have reached the maximum number of attempts. Please try again later."
       expect(page).to have_current_path(account_path)
     end
-
   end
 end