Take debug configuration into account when retrieving skin files

ATM we bundle only the minified JavaScript and CSS files in the XWiki WAR but I'm going to change that with XWIKI-13775 so we should modify getSkinFile to take into account the debug configuration when computing the path to the skin file. If minification is on it should look for the minified version. Otherwise it should look for the unminified version. This would ease a lot the debugging of an XWiki instance because adding ?minify=true to the URL will be taken into account by skin files also not just JSX and SSX.

Next step would be to modify the WebJar module to take into account the debug configuration so that we don't have to write IFs with $services.debug.minify in our pages or Velocity templates.