[imp] Semantic structure imporvements

## Happy articles have headers

Currently the post and page header elements lies outside of the article element, meaning they have no actual relation to the given article at all, beyond an implied visual relationship.

### Old friends, reunited.
This PR moves page and post `<header>` elements within the `<article>` element to restore the semantic relationship between the two.

### Testing
There are no visual differences in rendered output after this change. But the html5 outliner will be much happier.