One class per site-wide OPML file

[pfefferle]

Opml was the page OPML, the discovery links, the feed namespace and every file of the whole site in one class. This gives each of those files a class of its own: Directory, Recommendations and Following. The templates share their head now.

Each of them answers to its own value of ?opml on any URL: ?opml=directory, ?opml=recommendations and ?opml=following. That also works with plain permalinks, where the well-known paths do not. Opml::FILES maps the values to the classes, and render() just asks the right class to serve().

The directory and the recommendations are the same file for now, Recommendations just calls Directory. With a class each they can go separate ways later.

The well-known rule of the recommendations points to opml=recommendations now. The stored rule no longer matches, so the check for stale rules flushes once after the update. Until then the old rule lands on opml=directory, which is the same file.

Opml::directory() and Opml::DIRECTORY are gone, nothing outside the plugin uses them as far as I can tell.