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

app/controllers/users/sessions_controller.rb

@@ -1,5 +1,4 @@
 class Users::SessionsController < Devise::SessionsController
-
   private
 
     def after_sign_in_path_for(resource)
@@ -19,5 +18,4 @@ class Users::SessionsController < Devise::SessionsController
       stored_path = session[stored_location_key_for(resource)] || ""
       stored_path[0..5] == "/email"
     end
-
 end