Import some actual, serious content
Move posts to top-level, in per-year directories
Page.slug returns proper root-relative slugs

Fixes calculation of page URL slugs, so now all the slugs are guaranteed to
be relative to the root directory of the site basedir.
Fix handling of page dates

* The “date” metadata field is now properly parsed into a Date object.
* Comparison of dates is done in such a way that new pages/posts are
  listed first.