Update URL key

[jondashkyle]

Currently there are url, path and source keys.

• url: the formatted path (removes the parent content dir)
• path: the unformatted path of the true location (includes the parent content dir)
• source: used for remote content like https://example.com/

This should be updated to conform to Content State Schema.

• url: for pages this is formatted, for files this is unformatted
• source: used to define a remote location, makes it possible for other modules to fetch state

---

{
  '/about': {
    url: '/about'
  },
  'about/example.jpg': {
    url: '/content/about/example.jpg'
  }
}