comp AP gateway: convert pubsub item retractation to AP `Delete` activity:
when an item `retract` is received from a node from which an AP actor is subscribed, a
corresponding `Delete` activity is created and sent to AP followers.
Trying to retract an item from a virtual node of the gateway pubsub service now return a
`forbidden` stanza error (those nodes are handled on AP side and thus are not owned by
XMPP entities).
rel 367