component AP gateway: fix handling of XMPP comments authors: the gateway was supposing that comments where emitted from PEP of author. While this is the case for most blog posts, it's not for comments. Instead the component is now using `author_jid` which is retrieved by XEP-0277 plugin, and reject the item if the auhor is not verified (i.e. if `publisher` attribute is not set by XMPP service).