Disable 'Mark all as read' for notifications when everything is read

- This action needn't be performed when all notifications are read,
  hence disabling it is a good practice
- Added a link-disabled-color
- Used css pointer-events to disable clicking on the link

![image](https://f.cloud.github.com/assets/1003238/1047626/233f9694-1073-11e3-8578-08bc84c51cd8.png)