Show a yellowish note icon background for unapproved comments

![screen shot 2014-10-09 at 3 19 52 pm](https://cloud.githubusercontent.com/assets/789137/4585320/467c06ca-5004-11e4-8560-4e5d8d7d8b04.png)

To test, just set a comment to be unapproved from the notifications detail, then return to the list.

Fixes #2536 

cc @jleandroperez