Fixed issue where 'size()' is deprecated and still used and prevented making...

Fixed issue where 'size()' is deprecated and still used and prevented making action on multi-choice items.

flask_admin/static/admin/js/actions.js

 var AdminModelActions = function(actionErrorMessage, actionConfirmations) {
     // Actions helpers. TODO: Move to separate file
     this.execute = function(name) {
-        var selected = $('input.action-checkbox:checked').size();
+        var selected = $('input.action-checkbox:checked').length;
 
         if (selected === 0) {
             alert(actionErrorMessage);