Add case studies pages

[ericmjl]

Putting in a Lektor model for case studies. This enables us to write Markdown "case study reports" on models we've built.

Comes with the following features:

• Use cover.png or cover.jpg to add in a cover image for the gallery. Defaults to the Bayes image if there's no cover image.
• Visibility can be toggled.
• Authors are populated from a list on the admin page, though this list needs to be manually managed.
• Twitter Card fields get automatically populated with project title and summary sentence.

[OriolAbril]

Looks good, we can merge once you have time to add some guidance on the use of attachments

Thanks!