Set debug=False by default

[kevinlondon]

This changes the app to run with DEBUG=False unless the environment variable for FLASK_DEBUG has been explicitly set. Flask's docs include a note about how this should never be run on production with debug=True because it enables remote code execution. Patreon was supposedly just hacked because one of their test servers had debug=True enabled so I think it makes sense to keep it disabled by default in the interest of security.

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