Use .modal-open to prevent scrolling outside the modal

We should leverage Bootstrap's .modal-open that prevents the body from scrolling when a modal is open.

When working on this issue, I noticed that current modal implementation doesn't trigger CTools default dismiss behaviors, instead piggybacking on Bootstrap's modal javascript. But contributed modules might depend on CTools close behaviors to trigger side effects. I'm reverting that, making CTools modals independent from Bootstrap modals javascript-wise, but more integrated CSS-wise.