Handle prefix as generic.FormMixin does
Handle prefix as generic.FormMixin does

When using forms rendered as modals on a larger page, it is convenient
to use form prefixes passed by GET parameter.
Using the `get_prefix` method then allows us to return and handle the
prefix correctly:

```python
class PrefixedFormMixin(generic.edit.FormMixin):
    def get_prefix(self):
        return self.request.GET.get('prefix', self.prefix)
```

Ref:
http://ccbv.co.uk/projects/Django/1.11/django.views.generic.edit/FormMixin/#get_prefix

AndrewIngram/django-extra-views,AndrewIngram/django-extra-views