[JSON API] Relationships must exist

[cmars]

Relationship links must exist. For a relation like:

  relationships:
    authorized_apps:
      links:
        related: /path/to/authorized_apps

If there isn't a /path/to/authorized_apps, the relation is broken.

We need a rule that can check for this and flag broken relationships in our API.

Such a rule would likely need to operate at the compiled service or compiled SaaS level. It will also require link metadata in order to resolve to a route.