fix(discord): prompt only if permissions have changed

[lrstanley]

The current Discord oauth2 flow does not provide a prompt GET parameter during the redirect to Discord. As such, the default flow for Discord is to always prompt to validate the flow, even if the user has already previously approved the provided scopes. This PR ensures that prompt=none is provided, so Discord will only require a re-prompt if permissions have changed, or if there are elevated permissions being requested.

prompt controls how the authorization flow handles existing authorizations. If a user has previously authorized your application with the requested scopes and prompt is set to consent, it will request them to reapprove their authorization. If set to none, it will skip the authorization screen and redirect them back to your redirect URI without requesting their authorization. For passthrough scopes, like bot and webhook.incoming, authorization is always required.

I.e:

• If scopes haven't been previously approved, prompt.
• If sensitive scopes (bot & webhook.incoming) are used, ignore prompt parameter, and provide consent dialog.
• If all scopes have already been approved previously, auto-redirect back to application.

Should be a non-breaking change.

• closes discord oauth2 always forces authorization prompt (even if previously approved) #401