[NEWSLETTER] Settle RemovedInDjango18Warning in Django 1.7.x.

  *admin_forms.py: Creating a ModelForm without either the 'fields'
   attribute or the 'exclude' attribute is deprecated.

See [doc section](https://docs.djangoproject.com/en/dev/topics/forms/modelforms/#selecting-the-fields-to-use) for use of '\_\_all\_\_' where complete models are to be registered.