Refactor to work with OAuth 2.0

The previous version of this didn't actually seem to work, and it looked like a lot of the code has actually been implemented in Jared's `passport-oauth2` library. It also didn't seem to honor any of the weirdness about HubSpot's oauth implementation, like the fact that they don't have an authorization code step or utilize the verify callback.

I put myself as the author since it's such a big refactor, but I really don't care much. If you wanna own this repo going forward, I'd be happy to change it back :fried_shrimp: