Make modal backdrop static by default.

does not dismiss dialog accidentally if click on faded area.

Closes  #7403

IPython/html/static/base/js/dialog.js

@@ -88,6 +88,8 b' define(['
             options.keyboard_manager.disable();
         }
         
+        options.backdrop = options.backdrop || 'static';
+        
         return modal.modal(options);
     };