mod_pubsub_post: Allow specifying item id by a special mapping If `{http://jabber.org/protocol/pubsub}id` exists as an attribute at the top level then it is used, otherwise the previous behavior (.id if string)

This module discards typing notifications sent to a bare host JID,
preventing error replies to be sent. These errors are harmless but can
be annoying sometimes if your client shows them prominently.