Clean up RSVP button fns

The jquery element selector was just wacky and totally
dependent on the HTML structure.  Fix that by looking for
the div.event parent, and then the appropriate children
from there.

Correctly render the button, and include a utility function
to update the # of people going immediately.

Add a cancel RSVP hover state to provide more feedback
when you're going.
Update RSVP UI before ajax call

To make the UI more responsive when there's
latency, render the UI changes first, then make the ajax
call.

We're not doing good error handling on the page, so at
some point we really should settle on a policy and
implement 'error' functions on ajax calls.