Add no-text class to buttons with no text

![image](https://cloud.githubusercontent.com/assets/627891/2560591/9633961a-b7d4-11e3-9f83-0b56ec6e8050.png)
![image](https://cloud.githubusercontent.com/assets/627891/2560595/ace6bc8e-b7d4-11e3-8f21-9537e88f8313.png)
![image](https://cloud.githubusercontent.com/assets/627891/2560600/b6cc3a8a-b7d4-11e3-933f-a5c529e78211.png)
![image](https://cloud.githubusercontent.com/assets/627891/2560602/c1ddc682-b7d4-11e3-8963-cb3731b10684.png)

Note: This is the regex I used

    ack "<button class=[\"\']((?"'!'"no-text).)*[\"\'] .*?><i class=.*?></i></button>"

It has one more match, and that's the button to start bulk-selecting topics. The no-text class style doesn't look better there than the current one, so I left it alone.