database migration for Facebook long-lived user access tokens

In order to autopush opengraph objects to Facebook (issue #548), [extended access tokens](https://developers.facebook.com/docs/howtos/login/extending-tokens/) (which can be stored and are valid for 60 days) allow users to push opengraph objects even if they're not logged into FB at the time they upload a badge to their backpack.

Extend the users model to store and save these tokens and their expiration dates.