Make Celery non-optional

Advances AlekSIS-Core#359 (closed)

aleksis/apps/untis/management/commands/untis_import_mysql.py

@@ -5,4 +5,4 @@ from ...tasks import untis_import_mysql_current_term
 
 class Command(BaseCommand):
     def handle(self, *args, **options):
-        untis_import_mysql_current_term()
+        untis_import_mysql_current_term.delay()