Add release code name

I figured we're missing out on being cool and awesome by not having an
alphabetically based release code name like the big guys. This commit
fixes that. I've unilaterally decided on a theme of "$metal $bug"
because metals are kind of cool, and bugs, well, ...

cmd/syncthing/gui.go

@@ -355,6 +355,7 @@ func (s *apiSvc) getSystemVersion(w http.ResponseWriter, r *http.Request) {
 	w.Header().Set("Content-Type", "application/json; charset=utf-8")
 	json.NewEncoder(w).Encode(map[string]string{
 		"version":     Version,
+		"codename":    Codename,
 		"longVersion": LongVersion,
 		"os":          runtime.GOOS,
 		"arch":        runtime.GOARCH,