Stopped calling apps.get_model with only_installed=False.

ContentTypes are only created for installed applications.

ModelSignal actually needs has_registered_model since it's an
import-time lookup.
Used get_registered_models on non-master app registries.

Given that populate_models() is a no-op on non-master app registries,
this is strictly equivalent.
Simplified Apps.get_model and added AppConfig.get_model.

Documented them as public APIs.
Changed get_model to raise an exception on errors.

Returning None on errors led to unpythonic code, as the changes in this
commit show.