Check for Environment Variables before use

[burtonr]

When authentication is enabled (the default behavior), but the API_KEY environment variable is not set, the app crashes from an Error thrown by the jsonwebtoken library.

Need to check the existence of the environment variables to avoid unexpected errors.

Probably a startup check so the problem is known as soon as possible. Perhaps add a warning to the /settings response. This way the UI could get updated to show the message on load.

MONGO_URI is required. The app cannot function without it.
API_KEY is only needed if DISABLE_AUTH is not true
PORT is not required