mod_muc_moderation: Copy XEP-0421 occupant-id from retracted message Lets clients correlate the sender of whatever was retracted by moderators. Behavior limited to Prosody 0.12, otherwise there are no assurances of the origin of the occupant-id tag. Ref #1816

This module blocks configured queries that change server state.

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

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