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