Link tasks to their restarts

Closes #1711 (closed)

arkindex/ponos/api.py

@@ -223,7 +223,7 @@ class TaskRestart(ProcessACLMixin, CreateAPIView):
             raise ValidationError(
                 detail="Task's state must be in a final state to be restarted."
             )
-        if len(Task.objects.filter(original_task=task)) > 0:
+        if task.restarts.exists():
             raise ValidationError(
                 detail="This task has already been restarted."
             )