Support OAuth2 flows through the gui

[hlxid]

Description

All services that can use the OAuth2 implicit flow or PCKE for Oauth2 could be authenticated using OAuth in the GUI. It has the advantage that the user doesn't need to get any credentials and can just use the flow which saves time and makes using nodecg-io easier.

For services that need a OAuth2 flow which requires the client secret I would propose that the user has to provide its own application detail like client id and so on and for services that can use the implicit flow or PCKE like Twitch and Spotify we could have a registered application and publish the client id.

This would be implemented into the new GUI (#3) and therefore this is not really relevant to the current development and really in the future of this project.

References

OAuth discussion of ChatOverflow: codeoverflow-org/chatoverflow#168