Replace Basic/Extended options with a fieldset

I found that having the extended options only available when you edit an invitation to be quite horrible for user experience. In fact, it took me quite a while to realize that those options, and that important functionality was even available in this module. After looking at the code, it seems like the $rsvp must be available in order to switch between basic and extended.

Having this strange toggle mode, and risking losing pending content during a mode switch, I found it be to much better to simply place all of those extended options into a collapsed fieldset. So, what this patch does is removes the different modes and the toggleability, and places all of those options in a collapsed fieldset labeled as "Advanced options".