When I switched over to `flask run` entrypoint in b92391d177,
I forgot that the `if name==__main__` code no longer triggers.
So the SQLite in-memory database wasn't getting created for the example
app.
This moves the DB creation to a werkzeug/Flask hook that runs before the
first request to the app, so that the DB table is created when we query
it.
Also updated the test which worked fine previously, but this is more
idiomatic.
Drop `flask_script` in favor of Flask's native CLI:
* https://flask.palletsprojects.com/en/master/cli/
This also requires changing the tests so that `pytest` mocks the env var
`FLASK_ENV` so that the test app starts in development mode. Unlike
normal test apps, we _do_ want development/debug mode, in addition to
testing mode.
Dosenpfand