backend, delegation: update to XEP-0355 v0.5 (namespace bump) + disco: - delegation now uses namespace `urn:xmpp:delegation:2` - restored node metadata and made it work with PEP - `delegated` attribute is also set on recipient when available (needed to know when a disco query is delegated as the original stanza is lost by wokkel)

create table callbacks (
    service text not null,
    node text not null,
    uri text not null,
    PRIMARY KEY (service, node, uri)
);