Reorganize Facebook Authentication

This is work necessary for using large photos for posting on a user's timeline as per [bug 789043](https://bugzilla.mozilla.org/show_bug.cgi?id=789043).

- Replace the load_app view with FacebookAuthenticationMiddleware. A bunch of auth logic that was spread around multiple places has been consolidated into this middleware, and in general the code should be easier to follow.
- Move third-party cookie workaround to a separate middleware and use it on all browsers (now that Firefox is also moving to ban third-party cookies, we might as well apply it to all browsers).
- Add oauth_token field to user model.
- Remove FacebookDebugMiddleware, as it's unnecessary.