UC-145 Add FB disconnect button to prefs

RFCR @lizlux 

https://wikia-inc.atlassian.net/browse/UC-145
The solution looks good. A couple suggestions that are not deal-breakers but would add to the clarity of this code:
- `c` is not a very descriptive class name, but since `class` is a reserved word, I can see why you did it. Maybe `className` or `classString`?
- This method is getting a bit long, so it would be nice to split it out into a different function.