mod_conversejs: Show fallback text if converse.js fails to load (thanks MattJ) This can happen if javascript is enabled but 3rd party resources are blocked, or some other error happened.

This module blocks configured queries that change server state.

E.g. to make vCard storage read-only:

``` lua
readonly_stores = {
	vcard = { "vcard-temp", "vCard" };
}
```