Adjust transparent class of challenge buttons. 

Closes #4346. Also closes #4347 .

## Before 
(notice sliver of popover hiding just above leave button -- this if Firefox)
![screen shot 2014-12-05 at 3 16 02 pm](https://cloud.githubusercontent.com/assets/2916945/5323045/b3c12e2c-7c91-11e4-9e77-4538019228dd.png)

## After
![screen shot 2014-12-05 at 3 15 37 pm](https://cloud.githubusercontent.com/assets/2916945/5323048/b9b7a11c-7c91-11e4-858a-6bcf934f6da4.png)