Populated non-master app registries.

This removes the gap between the master app registry and ad-hoc app
registries created by the migration framework, specifically in terms
of behavior of the get_model[s] methods.

This commit contains a stealth feature that I'd rather not describe.
Stopped calling apps.get_model with only_installed=False.

ContentTypes are only created for installed applications, and I could
make a case for not returning a model that isn't installed any more.
The check for stale ContentTypes in update_contenttypes doesn't use
model_class.

ModelSignal actually needs get_registered_model since the lookup happens
at import time. I took this opportunity to perform a small refactoring.
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 required unpythonic error checking and was
inconsistent with get_app_config.

get_model was a private API until the previous commit, but given that it
was certainly used in third party software, the change is explained in
the release notes.

Applied the same change to get_registered_model, which is a new private
API introduced during the recent refactoring.
Removed the only_installed argument of Apps.get_models.

Refs #15903, #15866, #15850.