20 reasons to stay away from sections

Summary:
Ugh. I don't even like this. But it works.

Basically, this commit divides article editing/rendering into
"sections", which are visually indistinct, but separate editing
into horizontally-aligned sections. There will probably be more
options for sections in the future.

Test Plan:
 * Open the article.html editor
 * add, remove, and reorder sections
 * try preview and json modes

Auditors: alex