Add support for themes (fixes #1925)

gui/default/syncthing/core/upgradingDialogDirective.js

@@ -0,0 +1,7 @@
+angular.module('syncthing.core')
+    .directive('upgradingDialog', function () {
+        return {
+            restrict: 'A',
+            templateUrl: 'syncthing/core/upgradingDialogView.html'
+        };
+});