Add a `kind` hint to MF2 content

To make it easier to render the correct content type, as well as have links such as i.e. /kind/replies/, we want to have the kind taxonomy set up for our content.

This makes it possible to strip some painful duplication that was introduced as part of jvt.me#572, while also making it an easier experience for determining what content type an MF2 entry is.

As part of this, we're also wanting to pluralise the content type, as it makes the URLs more readable in line with what the content structure has them named as.

As part of jvt.me#689.