Implement OAuth2 for Visualizer Uploader

[jeffsf]

An upload-only scope is now available, thanks to Miha's work.

Use it and minimize potential damage due to exposure of credentials.

https://github.com/miharekar/decent-visualizer/commit/07f29b521426776a8fde9d1afa884b72518754af

[jeffsf]

It doesn't appear that Visualizer presently supports the kind of auth flow needed to obtain a token. It appears that only redirect-based flows are presently available. These would not be useful in a home-network situation without significant security risks.

https://auth0.com/docs/get-started/authentication-and-authorization-flow/which-oauth-2-0-flow-should-i-use