Make StockEvent a base class for Delivery and StockTaking

This helps to share code between the two entities and allows easier
extensions in the future.

app/controllers/deliveries_controller.rb

@@ -9,6 +9,7 @@ class DeliveriesController < ApplicationController
 
   def show
     @delivery = Delivery.find(params[:id])
+    @stock_changes = @delivery.stock_changes.includes(:stock_article).order('articles.name ASC')
   end
 
   def new