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/users_controller_spec.rb

@@ -1,7 +1,6 @@
 require "rails_helper"
 
 describe Management::UsersController do
-
   describe "logout" do
     it "removes user data from the session" do
       session[:manager] = { user_key: "31415926", date: "20151031135905", login: "JJB033" }
@@ -16,5 +15,4 @@ describe Management::UsersController do
       expect(response).to be_redirect
     end
   end
-
 end