Disable core plugin at startup

The reason for disabling it is related to another post I opened - Discourse v3.0.6 to v3.1.1 upgrade error - undefined method `register_bookmarkable' for Bookmark:Class

I know my install is not supported and that’s ok but the way the upgrades work is to deploy a new instance of discourse with the latest version. Due to this I need to disable Chat in the startup flow rather than post startup.

Hope that makes sense.