Add `exclude` meta attribute for `SocialAppForm`.

Django 1.7 requires ModelForms to have either a `fields` or an
`exclude` attribute in the `Meta` class.  Not putting either of the two
fields results in a

```
django.core.exceptions.ImproperlyConfigured: Creating a ModelForm without
either the 'fields' attribute or the 'exclude' attribute is prohibited;
form SocialAppForm needs updating.
```

exception.

Add an empty `exclude` Meta attribute to `SocialAppForm` to declare that
it should include all the model's fields.
Merge pull request #588 from lemuelf/django17_modelform

Add `exclude` meta attribute for `SocialAppForm`.