Remove django-celery dependency from active_status endpoint

[psychok7]

https://github.com/celery/django-celery is deprecated and it is also an unnecessary dependency.

We can redo the active_status endpoint without django-celery but using https://github.com/jezdez/django-celery-monitor/

So the new code would be added here: https://github.com/psychok7/django-celery-inspect/blob/master/celery_inspect/api/viewsets.py#L29 and it would instead import from:

from django_celery_monitor.models import WorkerState (and not from djcelery.models import WorkerState) and the check of installed apps would be django_celery_monitor instead of djcelery