Handle token revokation for authenticated users

[ksy36]

Once authenticated users revoke their Github token, it results in a 401 Bad Credentials error when sending requests to Github.

As a result, we're showing 500 error on certain pages (for example https://webcompat.com/issues, https://webcompat.com/issues/92175). It would be nice to log out such users once 401 error is received from Github.