fix the issue regarding the unban button

Meta: [Unban button for user that has never been banned?](http://meta.discourse.org/t/unban-button-for-user-that-has-never-been-banned/4941)

![Screenshot_21_03_13_00_58](https://f.cloud.github.com/assets/362783/283742/dde19f52-91ba-11e2-84ce-fcefcf9a3f58.png)

- [x] added `isBanned` property to deal with `null` response from the server
- [x] cleaned up a bit JS code
- [x] changed the *unban* logic: when a user is banned, we can **always** unban him (the previous behavior was that you could only unban a user if he wasn't an admin or a moderator)