See:
- https://github.com/Gidsy/django-threaded-messages/blob/8e79e4a3ea2abce6f53b4c1c6373d97ef1a2e333/threaded_messages/models.py#L23
- https://github.com/Gidsy/django-threaded-messages/blob/8e79e4a3ea2abce6f53b4c1c6373d97ef1a2e333/threaded_messages/models.py#L50
- https://github.com/Gidsy/django-threaded-messages/blob/8e79e4a3ea2abce6f53b4c1c6373d97ef1a2e333/threaded_messages/models.py#L60

So, when we undelete message, we should to reset deleted_at https://github.com/Gidsy/django-threaded-messages/blob/8e79e4a3ea2abce6f53b4c1c6373d97ef1a2e333/threaded_messages/views.py#L165