fixes for failed cron maintenance tasks

* update rails scripts for 5.2
* system dmails should be scoped to system
* fix broken url generator in forum notices

config/cable.yml

@@ -6,5 +6,5 @@ test:
 
 production:
   adapter: redis
-  url: redis://localhost:6379/1
+  url: <%= ENV.fetch("REDIS_URL") { "redis://localhost:6379/1" } %>
   channel_prefix: danbooru_production