Add page metadata #95

Each model with a view page should have a MetaTags subclass with a standard API that exposes image, title, description, etc. which should be passed to the template as meta so that the base template always knows how to display it in the header

http://ogp.me (for previewing links in telegram)

https://dev.twitter.com/cards/types/summary (for previewing links in Twitter)

https://support.google.com/webmasters/answer/79812?hl=en (standard meta tags for google, etc)

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#.iy9yd7v9f (slack)