Use basic auth instead of query params for auth in github-proxy

Created by: nicksnyder

Email from GitHub below:

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Hello there!

On February 6th, 2020 at 21:40 (UTC) your application (Sourcegraph.com github-proxy) used its client_id and client_secret (with the User-Agent Go-http-client/1.1) as part of a set of query parameters to access an endpoint through the GitHub API:

https://api.github.com/repositories/28159332

Please use Basic Authentication instead as using OAuth credentials in query parameters has been deprecated.

Depending on your API usage, we'll be sending you this email reminder at most once every 3 days. Just one URL that was accessed with a User-Agent combination will be listed in the email reminder, not all.

Visit https://developer.github.com/changes/2019-11-05-deprecated-passwords-and-authorizations-api/#authenticating-using-query-parameters for more information.

Thanks, The GitHub Team