WIP: DDH longtail summaries

Here's the current plan:
Summaries are added inside an HTML comment.
Add the title in an H2 tag with a `summary` class
Anything you want to show up in the longtail document is wrapped in a span tag with `summary-text` class.
The text can be written in HTML, markdown, or a mix of both.  If using mardown add `markdown="1"` class to the span.