rss.chat feeds now support WebSub

Andrew Shell's rssCloud server speaks both protocols as of its 4.0 release, over one shared list of subscribers -- so a reader that speaks WebSub can now follow an rss.chat feed in real time, the way an rssCloud reader always could. Two things make it work. Every feed we serve announces the hub in a Link header, which is the first place the spec tells a subscriber to look. And when a feed changes we tell the hub, right alongside the rssCloud ping we already send. It covers the per-user feeds, the everyone feed, and the subscription list at data/subs.opml -- WebSub works for OPML as well as RSS. (Server v0.6.14, on rss.chat and demo.rss.chat now.)