Add wrapper to "event is full" message.

#6068 

This PR adds a span wrapper to the "Sorry, event is full" text:
![selection_199](https://cloud.githubusercontent.com/assets/4497748/3076337/501e1b32-e3d0-11e3-91d0-ce47576d84d1.png)

Added a class to the span element of "register-event-full".