Work in progress - Make it possible to force the use of a given AppConfig subclass.

This behaviour is instantly deprecated, it's a migration aid not a new feature.

At present this misses:
- [ ] Documentation (incl. release notes)
- [ ] Adding `default_app_config` to all contrib apps
- [ ] Moving check registration into the default contrib apps configs
- [ ] Updating default project template, tutorial etc.
- [ ] Updating full test suite to avoid the new deprecation warnings