ref https://linear.app/ghost/issue/DES-1082/router-prototype
This task is about testing, figuring out pros and cons of React Router
compared to our current (custom) router, and what effort and risks are
involved in migrating to it.
ref https://linear.app/ghost/issue/DES-1021/create-posts-app
Part of establishing React patterns in Ghost is to build a well-defined
and fairly self-encapsulated app through which we can test assumptions
and define best practices. Our guinea pig is Post analytics for this
purpose. This PR creates a new React app (posts) using Shade (the new
design system).