Update registeration links for events when event is full.

#6068 

This PR addresses https://github.com/openscholar/openscholar/issues/6068#issuecomment-43641256 and now the registration link for events gets updated in case capacity is reached without reloading the page:

![selection_192](https://cloud.githubusercontent.com/assets/4497748/3038615/0239b2f8-e0d0-11e3-82d6-84796be44f35.png)