Fix bug 877423 - Clarify user ban in admin UI

https://bugzilla.mozilla.org/show_bug.cgi?id=877423
![ban1](https://f.cloud.github.com/assets/4639742/2375549/fffc9f0a-a86c-11e3-8c0d-119b7f713286.png)
![banned](https://f.cloud.github.com/assets/4639742/2375552/04a9d7d4-a86d-11e3-9d8a-f91da5da4d0a.png)