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 lets you block connections to any remote servers not on a
whitelist.

``` {.lua}
s2s_whitelist = {
    "example.org",
}
```