Move Utils::Misc::isUrl() function

All usage of this function gets to call Net::DownloadManager eventually.

src/base/utils/misc.h

@@ -71,7 +71,6 @@ namespace Utils
         };
 
         QString parseHtmlLinks(const QString &rawText);
-        bool isUrl(const QString &s);
 
         void shutdownComputer(const ShutdownDialogAction &action);