Add OpenGraph and Twitter Card metadata to pages

[pdehaan]

Minor nit, but will make the pages play much nicer with out other tools such as Activity Stream and Screenshots if users ever visit/screenshot a page on our site.

Refs:

• http://ogp.me/
• https://developers.facebook.com/tools/debug/
• https://dev.twitter.com/cards/types/summary
• https://cards-dev.twitter.com/validator

[pdehaan]

Dearest @johngruen,

Hopefully this message finds you well. If you want to throw a random mixture of og/twitter meta data into a comment, I can work on adding the appropriate <meta> tags to the .handlebars templates, running the various validators, and submitting a PR for your approval/bemusement/disgust.

Forever yours,
@pdehaan, ESQ