Usage in App is not working because of namespace issues

https://github.com/codingjoe/joeflow/blob/8d58695eeee2502d731fb5dde2491dba63e28c09/joeflow/models.py#L177C29-L177C29

If I use joeflow in an app in my project, the method to get the url namespace returns a wrong resulst.

it should be something like
```
return f"{cls._meta.app_label}:{cls.__name__.lower()}"
```

maybe somehow conditionally check if `cls._meta.app_label` exists

additionally one needs to specify the urls as follows:

```
path(
"whateveriwant/",
include(workflows.VeryCoolWorkflow.urls(), namespace="verycoolworkflow"),
),
```