Don't show the register link when the event has past.

#6781 
![image](https://cloud.githubusercontent.com/assets/1222368/5158135/82b4a912-733b-11e4-8c60-9f31ae2f2271.png)
![image](https://cloud.githubusercontent.com/assets/1222368/5158136/84ad9544-733b-11e4-988c-1e8c833dbeea.png)