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/management/sessions_controller_spec.rb

@@ -1,9 +1,7 @@
 require "rails_helper"
 
 describe Management::SessionsController do
-
   describe "Sign in" do
-
     it "denies access if wrong manager credentials" do
       allow_any_instance_of(ManagerAuthenticator).to receive(:auth).and_return(false)
       get :create, params: { login: "nonexistent", clave_usuario: "wrong" }
@@ -66,5 +64,4 @@ describe Management::SessionsController do
       expect(response).to be_redirect
     end
   end
-
 end