mod_pubsub_mqtt: Support atom_title payload type This commit adds the ability to publish and subscribe with arbitrary payload types. It has a breaking change, which is that topics are now of the form: HOST/TYPE/NODE Currently supported types are utf8_data, json and atom_title.

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.