gui: Use favicon as indication for status (fixes #1018)

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/3217

gui/default/index.html

@@ -14,7 +14,7 @@
   <meta name="viewport" content="width=device-width, initial-scale=1.0">
   <meta name="description" content="">
   <meta name="author" content="">
-  <link rel="shortcut icon" href="assets/img/favicon.png">
+  <link rel="shortcut icon" href="assets/img/favicon-{{syncthingStatus()}}.png">
   <link rel="mask-icon" href="assets/img/safari-pinned-tab.svg" color="#0882c8">
 
   <title ng-bind="thisDeviceName() + ' | Syncthing'"></title>