component AP gateway: convert XMPP mentions to AP: When a XEP-0372 mention is received, the linked pubsub item is looked after in cache, and if found, it is send to mentioned entity with `mention` tag added. However, this doesn't work in some cases (see incoming doc for details). To work around that, `@user@server.tld` type mention are also scanned in body, and mentions are added when found (this can be disabled with `auto_mentions` setting). Mention are only scanned in "public" messages, i.e. for pubsub items, and not direct messages. rel 369