Fix mollom in event registration

#3098 

We make sure the form it rebuilt, so if there's a validation error, we can keep the event registration fieldset open.

However moving the mollom inside the fieldset will require too much hacking in my opinion, as Mollom assumed the form element is in ``$form['mollom']``

![screenshot_6_26_13_10_28_am](https://f.cloud.github.com/assets/125707/707886/24996844-de32-11e2-9bd2-2c156144eed3.jpg)