Fix and reactivate Google Analytics in xwiki.org download form

GA is blocking the download form, this is why I deactivated it by commenting the lines calling GA in https://www.xwiki.org/xwiki/bin/view/XWikiOrgCode/GoogleAnalyticsWrapper .

To check why the form submit it blocked (an idea is that add blockers would be the cause), fix the issue and reactivate GA, as we need the data for marketing.