Make the Compatibility matrix visible to users installing an Extension and warn him if there's a problem reported with his versions

Right now we have a Compatibility matrix for Extension but it's only visible on extensions.xwiki.org. Thus when a user installs an extension from XWiki, he doesn't get to see it nor receive any warning if the extension has been reported to not work with his XWiki version. This leads to frustration and a bad image of the XWiki ecosystem.