Tidy all top-level HTML

This should make it easier to modify this content in the future, and to
see the changes in diff form.

I used https://github.com/w3c/tidy-html5 so elements like `<header>`
would be recognized. I used the following tidy config:

    break-before-br: yes
    char-encoding: utf8
    drop-empty-elements: no
    drop-empty-paras: no
    indent: auto
    show-body-only: auto
    wrap: 0
Merge pull request #16 from aroben/tidy

Run HTML Tidy on all content imported from WordPress