Allow to register a class based view

Add support to `adminplus.sites.register_view` for class based views

```python
class MyView(View):
    def get(self, request):
        ...
    def post(self, request):
        ...
    
admin.site.register_view('somepath', view=MyView)
```
This doesn't really need to be a lambda, since it doesn't depend on anything in this context. Probably a little more performant (and definitely sub-79-char lines) to make it a real function.