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.

Extends the users model to store these tokens.

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