Fix 2FA for users that receive code via SMS

hub initially tries to fetch user's existing OAuth token for hub if it
exists, and will prompt the user for a 2FA code. However those who
receive it via SMS would never get the code since a GET to GitHub's
Authorizations API purposely doesn't trigger an SMS. There's some
discussion as to whether this is an API bug for not, but in the
meantime, make a dummy POST request for the sole reason of triggering
the SMS.

Fixes #412, closes #435, closes #445