Add an XWiki.AccessDenied customization option

To further limit the need for customizing the standard XWiki skin (.vm files), and thus creating possible problems during upgrades, we need to offer a customization option for the Access Denied screen.

Most likely, the best option would be to allow overriding the default behavior (defined in accessdenied.vm) inside a wiki page, e.g. XWiki.AccessDenied, similarly to what we do for the other cases:

• XWiki.WikiDoesNotExist (in wikidoesnotexist.vm)
• XWiki.DocumentDoesNotExist (in docdoesnotexist.vm / templatedoesnotexist.vm)
• XWiki.AttachmentDoesNotExist (in attachmentdoesnotexist.vm)