Add check for the URLs setup of the XWiki instance

When the infra stack on which XWiki runs (frontal nginx or apache, tomcat, jetty) is wrongly configured, the URL auto-detection mechanism of XWiki may not be able to generate proper URLs from the wiki and thus some features may not work properly - this mainly concerns external URLs.

This issue is about creating a verification script for these settings in order to be able to quickly asses if an XWiki instance is configured correctly or not.