Clicking cancel in clone doc modal fix

Before, clicking cancel in the Clone Document modal found on the doc
editor page would redirect you back to previous page. This isn't what
you'd want or expect. Now it just closes the modal.

Closes COUCHDB-2509
assumption was that all our dismiss-buttons have this class and we could apply some general styling using this class. turned out my assumption was wrong: https://github.com/apache/couchdb-fauxton/search?utf8=%E2%9C%93&q=data-dismiss%3D%22modal%22