Fix Twitter Image Previews

[danirabbit]

Looks like twitter falls back to open graph, so there's no need to duplicate there. We do need the following:

• [x] add <meta name="twitter:card" content="summary_large_image" />
• [ ] add <meta name="twitter:creator" content="@twitterusername" />
• [x] add <meta property="og:description" content="page description goes here.">
• [x] remove <meta name="twitter:image" content="/images/preview.png">

Then check against https://cards-dev.twitter.com/validator