mod_rest: Allow empty @to to mean to=account is in normal XMPP Noticed an XXX in snikket-web-portal wrt needing to explicitly set @to in order to talk to 'self' services in Prosody, where to=nil is the same as to=account

This module blocks configured queries that change server state.

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

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