Once we transition to db.init_app, ensure library logs a warning or raises an exception if Flask is detected and init_app wasn't called, to avoid being backwards-compatible while still signaling how to fix

[dmalan]

Once we transition to db.init_app, ensure library logs a warning or raises an exception if Flask is detected and init_app wasn't called, to avoid being backwards-compatible while still signaling how to fix

[dmalan]

Nixing as unnecessary, since using thread-local storage.