Allow forcing using the wiki descriptor domain:port when generating an external URL

When you put a something like Apache or Nginx in front of XWiki you are supposed to configure it to indicate the source URL to XWiki. This source URL is then used by XWIki to generate consistent external URLs (when external URL are required).

If for some weird reason you cannot properly configure your frontend it should be possible to indicate that the source URL should never be trusted and fallback on the wiki descriptor instead.

The workaround right now if to use `xwiki.home` which has priority over the source URL (and the wiki descriptor) but only work for the main wiki.