Adds a lot of meta and Open Graph tags

This adds a bunch of open graph information, for social media sharing and SEO and stuff.

The Twitter card stuff, which would look [kinda like this](https://twitter.com/pepouille/status/511546164404289536), won't take effect unless you visit https://cards-dev.twitter.com/validator (after deploying this) and validate your card data. You can probably do it as either `@JoshData` or `@Mail-in-a-box`.