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/controllers/pages_controller_spec.rb

@@ -1,7 +1,6 @@
 require "rails_helper"
 
 describe PagesController do
-
   describe "Static pages" do
     it "includes a privacy page" do
       get :show, params: { id: :privacy }
@@ -20,7 +19,6 @@ describe PagesController do
   end
 
   describe "More info pages" do
-
     it "includes a more info page" do
       get :show, params: { id: "help/index" }
       expect(response).to be_ok
@@ -48,5 +46,4 @@ describe PagesController do
       expect(response).to be_missing
     end
   end
-
 end