Misc GUI enhancements and fixes

Changes:
 * Updated CKEditor from version 3 to version 4 (Known issue, Firefox 17 looses icons sometimes)
 * Updated jQuery UI from version 1.8.23 to version 1.8.24
 * Added feature to be able to change default months until a redirect expires
 * Changed error messages when trying to delete content with published content
 * Changed headline for adding images to add files
 * Fixed transparency for admin menu items in IE
 * Fixed a bug where content was added twice when using double click