mod_bookmarks2: Add new module. This is the result of hacking during the Stockholm XMPP Sprint, for compatibility with older clients only doing Private XML XEP-0048. This module shouldn’t be loaded at the same time as mod_bookmarks, as both implement Private XML to achieve their conversion.

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

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