mod_http_xep227: Initialize XEP-0227 XML mod_storage_xep0227 only writes if there is XML already for a user (it uses the presence of an existing <user> element as an indicator that an account exists, although technically this is not something Prosody itself does, so it's a little weird).

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

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