Convert backend to an api

The live website is server-side rendered. As described in &5, we're moving to a client-side rendered website in Elm (partially pre-rendered for progressive enhancement). So we need a way to surface the dynamic data to the Elm site.

While we could do something fancy like graphql, I'm inclined to make a simple REST api. We can continue using yesod for now; we just need to add API routes that return the relevant data, probably as JSON.