Allow #urPreview to scroll in the browser (fixes #2537)

This is the same issue as #2014/#2062. Bootstrap doesn't like having two dialogs
open at once: it marks the body has having no dialogs open when the first dialog
is closed, regardless of whether the second dialog is still open.

This means that scrolling doesn't happen properly, and the user cannot
scroll to the dialog's 'close' button.

Work around this by making sure the first dialog (the settings page) is fully closed
before the second dialog (usage preview) is opened.

gui/syncthing/core/syncthingController.js

@@ -1414,10 +1414,13 @@ angular.module('syncthing.core')
         };
 
         $scope.showURPreview = function () {
-            $('#settings').modal('hide');
-            $('#urPreview').modal().on('hidden.bs.modal', function () {
-                $('#settings').modal();
-            });
+            $('#settings').modal('hide')
+                .one('hidden.bs.modal', function() {
+                    $('#urPreview').modal()
+                        .one('hidden.bs.modal', function () {
+                            $('#settings').modal();
+                        });
+                });
         };
 
         $scope.acceptUR = function () {