Define built-in models via JSON

Refactor LoopBack built-in models to follow the convention supported by loopback-boot and loopback-workspace. This allows Strong Studio and generator-loopback to introspect built-in models.

 - `Model` and `PersistedModel` are staying in the `lib`, since the are tightly coupled with loopback core.
 - `AccessContext` is staying in `lib` too, since it's not a model.

Depends on https://github.com/strongloop/loopback-datasource-juggler/pull/323

**BLOCKERS**
 - [x] #640 Move models to common/models
 - [x] https://github.com/strongloop/loopback-explorer/pull/59 Support multi-line array `description` and `notes`