Use persistent ids for content, not paths

This allows us to handle the upcoming draft content, and also to survive post-launch url changes. 

Collections now look like:
```
{
  "live" : [
      {
        "id" : "internal-code/content/432607259"
      }, {
        "id" : "internal-code/content/432610156"
      }, {
        "id" : "internal-code/content/432595839"
      } ... etc
```