`source:account` moved to where the spec says it goes

Our feeds had been putting a source:account element inside every item. The source namespace defines it as a channel-level element — it says whose feed this is, not who wrote an item — and at the item level it was redundant anyway: in the mixed feeds, every item already carries a <source> element with the author's feed URL and name. As of today, each user's own feed says it once, in the channel: <source:account service="rss.chat">dave</source:account>. The items carry it no more. The everyone feed and the comments feeds have no channel account — they have many authors, and their items keep <source> for per-item attribution. If you've been reading the item-level element, read the channel-level one (single-author feeds) or <source> (mixed feeds) instead. Already-published feeds keep the old shape until they next rebuild. The report that got this fixed is issue #9. The channel-level support went into daverss, the shared RSS-building package, as a new optional setting.