Updating generic views

Adding `HybridListView` and `HybridDetailView`, which work much like the standard `ListView` and `DetailView` but are able to serve additional content types depending on the request.

```python
# views.py
from mozbadges.views.generic.detail import HybridDetailView

class ModelDetailView(HybridDetailView):
    model = Model

model_detail = ModelDetailView.as_view()
```
```python
# urls.py
url(r'^.../model(?:/|.json|.xml)$', views.model_detail)
```

Also moved `placeholder_view` from `mozbadges.utils.views` to `mozbadges.views` - at some point we can kill it entirely.