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/base_controller_spec.rb

@@ -1,9 +1,7 @@
 require "rails_helper"
 
 describe Management::BaseController do
-
   describe "managed_user" do
-
     it "returns existent user with session document info if present" do
       session[:document_type] = "1"
       session[:document_number] = "333333333E"
@@ -23,5 +21,4 @@ describe Management::BaseController do
       expect(managed_user.document_number).to eq "333333333E"
     end
   end
-
 end