Allow supporting XWiki deployed as ROOT webapp without having to set xwiki.webapppath

The issue XWIKI-10339 which was fixed in 6.1M1 introduced a regression leading to the Watchlist sending invalid emails when XWiki is deployed as a ROOT webapp...

In XWIKI-11843 a stopgap solution was found by allowing users to configure their xwiki.cfg file to explicitely set up the following configuration when deploying XWiki as ROOT:

xwiki.webapppath=

Now, it would be nicer if XWiki could automatically detect the webapp context used. This is possible starting with Servlet 2.5 thanks to http://docs.oracle.com/javaee/6/api/javax/servlet/ServletContext.html#getContextPath()

Thus this issue depends on XWIKI-9358 and once that issue is fixed, we'll be able to save the webapp context in our XWikiServletContextListener so that it's available to XWiki.getWebAppPath() and to ExtendedURLURLNormalizer.normalize().