Feature/update check (#1232)

Periodically fetch the latest available version, and the UI will shows a new menu for the download link. It checks both the daemon version and the UI version.

version/url_linux.go

@@ -0,0 +1,6 @@
+package version
+
+// DownloadUrl return with the proper download link
+func DownloadUrl() string {
+	return downloadURL
+}