Add muted class when no food is in inventory

Fix for issue #4405. Adds `muted` class if no food is in the users inventory.

**Before:**
![before](https://cloud.githubusercontent.com/assets/278775/5560214/093fb2a8-8d2f-11e4-8bf8-0a61091a0ca7.png)

**After:**
![after](https://cloud.githubusercontent.com/assets/278775/5560215/0cd36036-8d2f-11e4-83bf-75d475f75d7e.png)