Remove home-cooked modal, replace with jQuery UI modals

So we've been using two flavors of modals on MDN:  the pieced-together modal within the demo studio and then the jQueryUI modal.  This converts the demo studio to using the jQuery UI modal.  To test:

- [x]  Report a problem on the demo feature page and from the launch bar.
- [x] Delete a comment
- [x]  Delete a demo