Avoid setting global config variables that are not otherwise referenced

[brassy-endomorph]

https://github.com/scidsg/hushline/blob/75ee6e0e58e71f014aedf4381dbd377d38691de7/app.py#L63-L66

We can directly create the Flask-SqlAlchemy config from env vars rather than creating temporary vars. We can also just set the config directly via an env var called DB_URI or something.

We may however want to create our own Config python object that pulls env vars and pre-processes them for us then dumps them into Flask's config after munging.