added ability to link recipes to ingredients

cookbook/models.py

@@ -127,6 +127,7 @@ class Unit(models.Model):
 
 class Ingredient(models.Model):
     name = models.CharField(unique=True, max_length=128)
+    recipe = models.ForeignKey(Recipe, null=True, blank=True, on_delete=models.SET_NULL)
 
     def __str__(self):
         return self.name