feat(auth / sync): add github app auth and keep description capabilities

[kema-dev]

GitHub App auth / Description syncing

Motivation

• It was not (to my knowledge) possible to use a GitHub App for authentication
• Description was not synced

Implementation

• Removed Users API calls when using a GitHub App (thus using a ghs prefixed token, see here)
• Added description syncing
• Added keep-description flag to make it possible not to override destination repo's description. This is also useful to prevent rate-limiting, as description fetching is making unauthenticated API requests.