Redirect to level editor only after modal hidden event is triggered. Fixes #438 and #439.

The `modal('hide')` function does not wait until the modal is fully hidden before the Backbone's `router.navigate` is triggered. Hence there was a transparent modal backdrop `<div class="modal-backdrop fade in"></div>` that remains and was covering the whole page, causing the whole page to be unresponsive.

To fix this, a event listener for the modal hidden event was added and the redirecting to the editor page only happens when the `hidden.bs.modal` event was triggered.

Fixes issues #438 and #439.