Add a warning when replacing a page with a redirect

Users that are trying to correct a partial move of large spaces (see also XWIKI-16919) can end up easily replacing pages with a redirect (a page with no content and a redirect object resulted after a move / rename action with "Create an automatic redirect " checked). As a result, the "corrected" pages will display "The page isn’t redirecting properly".

The rename dialog should display a warning before overriding the "Original page" with an actual redirect page proposed for "Replace with".