Add unfurl metadata to major URLs

[themightychris]

For projects, updates, blog posts, and buzzes: add metadata tags to the page templates so that Slack and Discourse unfurl links into quality previews

[schlos]

Infos:

• https://metatags.io/
• https://css-tricks.com/essential-meta-tags-social-media/
• 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
• https://secure.phabricator.com/T13018