gui: Have static link to favicon, for bookmarks etc (fixes #7638) (#8850)

gui/default/index.html

@@ -12,7 +12,8 @@
   <meta charset="utf-8"/>
   <meta http-equiv="X-UA-Compatible" content="IE=edge"/>
   <meta name="viewport" content="width=device-width, initial-scale=1.0"/>
-  <link rel="shortcut icon" href="assets/img/favicon-{{syncthingStatus()}}.png"/>
+  <link rel="shortcut icon" href="assets/img/favicon-default.png" type="image/x-icon"/>
+  <link rel="shortcut icon" href="assets/img/favicon-{{syncthingStatus()}}.png" type="image/x-icon"/>
   <link rel="mask-icon" href="assets/img/safari-pinned-tab.svg" color="#0882c8"/>
 
   <title ng-bind="thisDeviceName() + ' | Syncthing'"></title>