Allow non-oscar dashboard application

After @maikhoepfel fixing of dashboard navigation, Oscar do not accept dashboard applications that are not in Oscar.

Example: If my project has a dashboard application for managing a blog (not part of Oscar), I'll get the following error:

```
ImproperlyConfigured: Please follow Oscar's default dashboard app layout or set a custom access_fn
```

This is a severe issue and it is a release blocker for 0.6