Change to Categories Manager Icon

Suggest changing 'categories manager' icon to fa-object-group so it is more distinct to 'manage files':
![untitled](https://user-images.githubusercontent.com/1744687/32510641-2d5c4180-c3e9-11e7-9a90-b06ec377ae6c.png)

