Use basic authentication for client_id/client_secret usage with GitHub API authentication

Use basic authentication for client_id/client_secret usage with GitHub API authentication

Part of https://gitlab.com/gitlab-org/gitter/webapp/-/issues/2442

Dev notes

• https://developer.github.com/apps/building-github-apps/understanding-rate-limits-for-github-apps/
• https://developer.github.com/v3/#increasing-the-unauthenticated-rate-limit-for-oauth-applications