Refactor Note REST API endpoint

This is a working implementation for the changes outlined in #149 (closed). I’ve added some tests that can be executed with

poetry run ./manage.py test

You may want to integrate these as a CI Job.

.gitignore

@@ -3,5 +3,6 @@ db.sqlite3
 .mypy_cache
 *.pyc
 .pytest_cache
+.cache/
 static/
 steering_data_model.png