allow importing as a script without GUI requirements installed

[terencehonles]

This change updates the script initialization to not fail on import errors, to finish without checking if --no-gui or --external-auth has been passed in the command-line, and instead raises an error when the application is initializing if running in GUI mode and the requirements are not met.

Minor changes also included:

• The --no-gui option is now stored as gui and defaults to True which changes comparisons to not be double negatives but retains the existing behavior.
• The App instance is allowed a single argument which is the command-line to parse, which allows starting the app with arguments from an import.

[terencehonles]

This will also allow packaging without all the requirements and this check https://github.com/simonrob/email-oauth2-proxy/pull/199/files#diff-8068eda2feddbc61595ced559aa73f8502813cf5cb1190a21e111ab1e8d19e88R71