mod_pubsub_subscription: support subscribing from a bare JID Allow subscribing from a bare JID on the component instead of only the component host, useful for subscribing to whitelist access model nodes that want to see a particular JID in the from.

This module helps make BOSH and WebSocket connection endpoints
discoverable via the HTTP method described in
[XEP-0156](https://xmpp.org/extensions/xep-0156.html#http).