HTML housekeeping

MIT has (and Chicago soon will have) a layout that is sensitive to correctly closed `<div>` tags. This is the motivation for this set of changes.

In addition I have some desire to uniformize our heading usage--e.g. not using `<h2>` in a page if we haven't used `<h1>` yet, or perhaps making use of `{% block content_title %}`; but that could require messing with lots of chapters' layouts so maybe I should hold that thought.

My notes on the scans that discovered these:

* [missing tags](https://github.com/pteromys/ESP-Website/wiki/Closing-Tags---Missing-Tags-Scan)
* [extra tags](https://github.com/pteromys/ESP-Website/wiki/Closing-Tags---Extra-Tags-Scan)