Change event registration link if event is full.

#6068 

WIP:

Now there's a link to register to the event in "Teaser" view mode:
![selection_181](https://cloud.githubusercontent.com/assets/4497748/3012114/229bad9a-df35-11e3-87db-a0e9e403e027.png)


When event capacity is reached instead of no link there's a text:
![selection_182](https://cloud.githubusercontent.com/assets/4497748/3012116/288310f4-df35-11e3-9046-bd42c79b2309.png)