Update for latest Flask version (#4338)
Update for latest Flask version (#4338)

- Remove the not-supported eager-loading option from the flask run command
- Replace the deprecated FLASK_ENV command with FLASK_DEBUG
Dallinger/Dallinger,Dallinger/Dallinger,Dallinger/Dallinger,Dallinger/Dallinger,Dallinger/Dallinger