Add support for pubsub#notify_sub for Pubsub and PEP Nodes

[edhelas]

As documented in the XEP-0060, pubsub#notify_sub allows a node owner to be notified about new subscribers and unsubscribes.

This feature not currently implemented in ejabberd, for Pubsub and PEP Nodes.

Having this feature in Movim will allow users to get notified about a new follower on their account.

[edhelas]

Related issue in Prosody https://issues.prosody.im/1972