Instead of having them under /posts/ we can instead store them in their
own content type, with a better directory and URL structure.

This requires updating any existing links to them on the site, using
`ref`, but also so any external links will work, too.

We can also remove `tags`, `series` and licensing information as they're
now handled by the `week-notes` folder, and remove the `slug`, which no
longer makes sense.  We add the `type: posts` to make sure that Hugo
treats them as articles.

Closes #1006.