Improve OpenGraph data

[harrislapiroff]

Let's get those social media previews!

• [x] Default image
• [ ] Algorithmically generated image?
• [x] Basic OG tags (title, description, etc.)

[harrislapiroff]

I've added the basic tags and default image, but there's still more to go!

• [ ] Algorithmically generated default image?
• [ ] Tags on non-article pages?
• [ ] Publication date tags (see: https://ogp.me/#type_article)
• [ ] Find a higher quality image size by default