Use standard Github oauth for authentication

Following up on https://github.com/fluencelabs/dev-rewards/issues/48, it seems to be impossible for some developers to authenticate themselves by decrypting with `age`, particularly for those who sign with a yubikey.

Using [oauth](https://docs.github.com/en/apps/oauth-apps/building-oauth-apps/authorizing-oauth-apps) to authenticate against Github is a lot more user-friendly, more developers would be able to claim their rewards (those that now can't) and also would seem less dangerous for those worried about exposing their private keys.