Fix invalid HTML caused by extra `</article>` closing tag.
Add `<section>` HTML for opt-in campaign updates.
Revert "Include `css/main.css` as embedded style sheet Because Tumblr. (I know.)"

This reverts commit 90b5ec2ff1a68581096a56ffb6dfce9e6ceb2b2c.
This removes the embedded stylesheet for the Tumblr theme, which should
only be present in the `tumblr` branch.
Include `<link>` to local `css/main.css` stylesheet.
Fix broken HTML; remove `</a>` tag for `<a>` element that is not open.
Styling and additional semantic classes for "campaign updates" section.