mod_http_oauth2: Add a special "xmpp" scope that grants the users' default role This will be the first step towards defining a standard set of XMPP scopes. "xmpp" behaves as an alias for the user's default role, so that the client does not need to know about the various prosody:* roles.

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

``` {.lua}
modules_enabled = {
    -- other modules --
    "s2s_whitelist",

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