Fix github deprecation warning (for github login support)

Got an email from GitHub:

[GitHub API] Deprecation notice for authentication via URL query parameters

Hi @souramoo,

On July 30th, 2020 at 19:20 (UTC) your application (Commento) used an access token (with the User-Agent Go-http-client/1.1) as part of a query parameter to access an endpoint through the GitHub API:

https://api.github.com/user/emails

Please use the Authorization HTTP header instead as using the `access_token` query parameter is deprecated.

Depending on your API usage, we'll be sending you this email reminder on a monthly basis.

Visit https://developer.github.com/changes/2020-02-10-deprecating-auth-through-query-param for more information about suggested workarounds and removal dates.

Thanks,
The GitHub Team

This commit fixes this API deprecation to allow commento to keep working with github OAuth past May 5, 2021 at 16:00 UTC. (Although it seems this API change has already taken place?)

Fixes #278