Add metadata to the page to help with Slack/Twitter/Facebook previews

[fdb]

This requires something like #16 to be implemented first.

Here's a guide to "unfurling":
https://medium.com/slack-developer-blog/everything-you-ever-wanted-to-know-about-unfurling-but-were-afraid-to-ask-or-how-to-make-your-e64b4bb9254