dotenv support

[movexig]

Adds support for dotenv files, primarily for local development (though live can use it too).

• adds python-dotenv as a requirement
• unpinned several requirement versions to make vf build properly on later python versions
• cleaned up settings and made all settings read directly by votefinder have the VF_ prefix
• settings now a module with a helper class for more convenient type management of stuff from env
• logging can now be enabled regardless of whether DEBUG is set