Form admin inline  extra inlines

Fix this use case:

```python

class FormDefinitionTaskAdminInline(admin.StackedInline):
    model = FormDefinitionTask

class MyFormDefinitionAdmin(FormDefinitionAdmin, reversion.VersionAdmin):
    inlines = [FormDefinitionTaskAdminInline] + FormDefinitionAdmin.inlines

admin.site.unregister(FormDefinition)
admin.site.register(FormDefinition, MyFormDefinitionAdmin)
```