Specify Flask logging level via CLI

[jdeanwallace]

While reviewing #1666, @jotaen4tinypilot suggested we suppress some of the Flask logging when running Flask in CLI commands. Example of logs to suppress:

$ FLASK_APP=app/main.py flask cli streaming-mode
2023-10-30 20:08:12.626 app.main        INFO Starting app
2023-10-30 20:08:12.658 db.store        INFO Loading database migrations from /opt/tinypilot/app/db/migrations/*.sql
2023-10-30 20:08:12.660 db.store        INFO Read 7 database migrations from disk
H264

It seems like we'll need to allow app/main.py to dynamically set the log level based on an environment variable:

tinypilot/app/main.py

Line 30 in 959a5a8

root_logger.setLevel(logging.INFO)