title: Django admin - Foreign key popup '+' missing when overriding admin form field
body: <p>I've set up a basic django site using the automatic admin, and was using the '+' button to add foreign keys (plants) while adding an Ingredient. However, the plants weren't ordered so I've had to override the form field as show below. </p>
  
  <p>It seems though that doing this means the '+' button which opened the popup has gone missing - is there a simple way to reinclude this, or a better way I should be ordering the foreign key dropdown? Thanks.</p>
  
  <pre><code>class IngredientAdminForm(forms.ModelForm):
      plant = forms.ModelChoiceField(queryset=Plant.objects.order_by('name'), required=False)
  
      class Meta:
          model = Ingredient
  
  
  class IngredientAdmin(admin.ModelAdmin):
      list_display = ('name', 'description', 'plant')
      form = IngredientAdminForm
  </code></pre>