Make `:nuzzle/updated` a required page entry key

[stelcodes]

The Atom spec requires every entry have both a title and an updated timestamp. Since Nuzzle is now aiming to integrate tightly with Atom, all page entries must have an updated value. This will require more work for the user but it will also produce great Atom feeds and sitemaps since both use the updated value.

[stelcodes]

Actually this is a bad idea, adding timestamps to every page is way too much friction for a simple site.