Update GitHub sign-in API calls

[SUPERCILEX]

This sucks:

Hi @SUPERCILEX,

On February 24th, 2020 at 11:04 (PST), your application (Robot Scouter) issued a request using the deprecated form of OAuth Application API that includes access tokens in the URL path. GitHub has deprecated these endpoints and replaced them with a version that accepts access tokens in the request body.

The deprecated endpoints will be removed on July 1st, 2020 at 4:00 PM UTC.

Please visit https://developer.github.com/changes/2020-02-14-deprecating-oauth-app-endpoint for more information about suggested changes, brownouts, and removal dates.

Thanks, The GitHub Team