Create Django apps with models, URLs, and view stubs

* Keep `core` app small (with no UI, and just the ubiquitous User and Group models)
* Create topic-specific apps for handling groups of models and the URLs that service them
* Name all URLs (usually the view name)
* Use a `urls` module where an app handles URLs with more than one prefix
* Create view stubs for all URLs and import where referenced
* Create stubs for decorators and import where referenced
* Add apps to `settings.LOCAL_APPS`
* Distribute all models from `doc/draft-models.py` and all URLs from `doc/draft-urls.py`

Note that it all loads in our `app` VM using this command:
     `envdir /etc/nyc-trees.d/env ./manage.py shell`

To grok the URL scheme see https://github.com/RickMohr/nyc-trees/blob/make-apps-and-urls/src/nyc_trees/nyc_trees/urls.py