Current implementation of `app.model(modelName, settings)`
works as a sugar for model creation. In 3.0, this is
not supported anymore. This implementation reports an
error when sugar is used for model creation.
Includes:
- Updated app.model() method
- Fixed test cases reflecting the change
- `loopback.registry` is now a true global registry
- `app.registry` is unique per app object
- `Model.registry` is set when a Model is created using any registry method
- `loopback.localRegistry` and `loopback({localRegistry: true})` when set to `true` this will create a `Registry` per `Application`. It defaults to `false`.