Add an icon for Safari's pinned tabs

Safari has its own standard for handling icons for pinned tabs,
which requires a black-and-white .svg and a special tag.
Without using this, pinning a tab to localhost will show just
a blank square, instead of a pre-generated letter.

gui/index.html

@@ -15,6 +15,7 @@
   <meta name="description" content="">
   <meta name="author" content="">
   <link rel="shortcut icon" href="assets/img/favicon.png">
+  <link rel="mask-icon" href="assets/img/safari-pinned-tab.svg" color="#0882c8">
 
   <title ng-bind="thisDeviceName() + ' | Syncthing'"></title>
   <link href="vendor/bootstrap/css/bootstrap.min.css" rel="stylesheet">