client: ignore readonly models

Ignore readonly loopback built-in models as the GUI is not ready to handle them yet.

See https://github.com/strongloop/loopback-workspace/pull/160 and https://github.com/strongloop-internal/scrum-loopback/issues/34.

Support for readonly models is covered by this item: https://github.com/strongloop/strong-studio/issues/431

/to @ritch please review