add an intermediate class between order and article

Because we need additional fields in an order it's necessary to add an
additional class where the information gets stored.

django/didgeridoo/webshop/models.py

@@ -94,11 +94,21 @@ class Picture(models.Model):
 
 class Order(models.Model):
     user = models.ForeignKey(User, on_delete=models.CASCADE)
-    article = models.ManyToManyField(Article)
-    order_status = models.ForeignKey(OrderStatus)
+    article = models.ManyToManyField(Article, through='OrderPosition')
+    status = models.ForeignKey(OrderStatus)
     date = models.DateTimeField(default=datetime.datetime.now())
 
 
+class OrderPosition(models.Model):
+    article = models.ForeignKey(Article, on_delete=models.CASCADE)
+    order = models.ForeignKey(Order, on_delete=models.CASCADE)
+    amount = models.FloatField(max_length=5)
+    price_in_chf = models.DecimalField(max_digits=19,
+                                       decimal_places=2,
+                                       validators=[MinValueValidator(
+                                            Decimal('0.00'))])
+
+
 class ShoppingCart(models.Model):
     name = models.CharField(max_length=200)
     user = models.ForeignKey(User, on_delete=models.CASCADE)