Move away from using `<article>` for non-syndcated content

From [the spec about sections](http://www.w3.org/html/wg/drafts/html/master/sections.html#the-section-element):

> Authors are encouraged to use the article element instead of the section element when it would make sense to syndicate the contents of the element.

Instances where it does not make sense to syndicate the content, sections should be used. Out of the box, only posts and comments are syndicated.

Please let me know what you think of the class name changes and whether they make sense in the current proposal.