Make chatroom disableable.

The change to make chat rooms opt-in for "web" connections is only marginally useful.  For the case of creating just API endpoints, there should be no need to do any work at all related to chat rooms.  This change adds a `general.chatEnabled` option to config.js that allows turning off all chat messaging capabilities.  By default (including if the option is omitted from config.js) chat will be enabled and will work as it does today.