Stop relying on the application server for XWiki#getRequestURL

As shown by XDOCKER-191 and OIDC-112 depending on the application server, and it's setup it's not always very reliable, and HttpServletUtils#getSourceURL which an XWiki implementation which supports a lot of HTTP proxy headers seems to work well in those cases.

In general, relying on the application setup always cause various issues and makes XWiki harder to install, so we should try to avoid it as much as possible.