Always import Celery config if available

aleksis/core/__init__.py

 import pkg_resources
 
+try:
+    from .celery import app as celery_app
+except ModuleNotFoundError:
+    # Celery is not available
+    celery_app = None
+
 try:
     __version__ = pkg_resources.get_distribution("AlekSIS").version
 except Exception: