Page deletion protection should warn when clicking the delete menu option

Right now the process is:

• User clicks on delete in the menu
• The user has the option to refine the deletion by selecting children and seeing backlinks
• He then clicks "Yes" to proceed
• Then during the job execution, a warning is shown if some pages belong to extensions

Issues:

• It would be better that the user sees the potential problems on the first screen since that screen is there to let the user decide what to delete already.
• Since the job progress is already started, the user doesn't know if some pages have already been deleted or not when he's asked again to select the pages to delete (if some belong to an extension).