mod_mam_archive: remove invalid disco#info feature The feature which was previously added to disco#info was in fact specified to be used for stream features only (see XEP-0136 section 11). Emitting it in disco#info is weird at best and breaks stuff at worst.

This module lets you block connections to remote servers at the s2s
level.

``` {.lua}
modules_enabled = {
    -- other modules --
    "s2s_blacklist",

}
s2s_blacklist = {
    "proxy.eu.jabber.org",
}
```