Add wrapper to mark an optional Celery task

This allows code that does not need Celery to also be run synchronously, e.g.
if the administrator does not want or cannot run Celery.

aleksis/core/util/core_helpers.py

@@ -102,3 +102,22 @@ def has_person(obj: Union[HttpRequest, Model]) -> bool:
             return False
 
     return getattr(obj, "person", None) is not None
+
+
+def celery_optional(orig: Callable) -> Callable:
+    """ Decorator that makes Celery optional for a function.
+    
+    If Celery is configured and available, it wraps the function in a Task
+    and calls its delay method when invoked; if not, it leaves it untouched
+    and it is executed synchronously.
+    """
+
+    if hasattr(settings, "CELERY_RESULT_BACKEND"):
+        from ..celery import app  # noqa
+        task = app.task(orig)
+
+        def wrapped(*args, **kwargs):
+            task.delay(*args, **kwargs)
+        return wrapped
+    else:
+        return orig