Display error when the user try to register to a booked event.

#4906 
I handle this issue by hiding the link when the event got fully booked. But i also made fallback that if the user manage to open the modal and the event is fully booked he will get the next screen:
![selection_004](https://f.cloud.github.com/assets/1222368/1949770/ddd79dc2-811f-11e3-90dc-91896ad515f3.png)