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 not fully hidden before the Backbone router.navigate is triggered. Hence there was a transparent modal backdrop `<div class="modal-backdrop fade in"></div>` covering the whole page, causing the whole page to be unresponsive.

Hence I added an event listener for the modal hidden event and redirected to the editor page only when the `hidden.bs.modal` event was triggered.

Fixes issues #438 and #439.