Simplify JS

* Render an image inside each `.seat` and show/hide it with `.seat-selected`. In this case, it's more efficient to toggle a class than to move dom pieces around.

* Remove the logic to deselect a seat since it isn't valid to have an attendance without a seat. Feel free to add that back if you like.

* Add `require_tree .` to `application.js`. This will require all files in `app/assets/javascripts` and compile them in production.

I might recommend you start looking at JS frameworks like Backbone, Angular or Ember. You clearly have some awesome jQuery chops, and I think it would be beneficial for you take what you know to the next level.