Dependency graph API

[stevenle]

Description

For sites that make heavy use of reference fields, we should provide a way to create and fetch a dependency graph that tells the caller what reference dependencies are required for one or more docs.

Within the batch request system, the API can intelligently figure out which docs requested via get() will be available via list() and avoid duplicate fetches.

There should be some post-publish hook that generates this graph so it can be cached and not re-created on every request.

The CMS UI can also use this graph to provide warnings if reference docs have "draft" content when the main doc is being published.