UC-138 Update the user_fbconnect table and code that accesses it

Changes to record all the data necessary to disambiguate Facebook users now that we don't get a universally unique ID.

https://wikia-inc.atlassian.net/browse/UC-138
as above, perhaps: 

```
ALTER TABLE user_fbconnect 
  ADD UNIQUE KEY (user_id, user_fb_app_id),
  ADD UNIQUE KEY (user_fbid, user_fb_app_id);
```