Improve wrapping of the unsubscribe button for subscriptions list

Possible fix for openaustralia/publicwhip#991.

Unsubscribe button stays floated to top-right instead of wrapping around, with a new small 'X' icon:
![image](https://cloud.githubusercontent.com/assets/103446/5422814/65a6b27c-82e3-11e4-8a1d-cf513d6cdef9.png)

On smaller screens, the 'unsubscribe' button switches to only the 'X' icon:
![image](https://cloud.githubusercontent.com/assets/103446/5422818/9d37520a-82e3-11e4-966a-f26b46b49449.png)