Generate a random API key on initial setup (fixes #1118)

Also makes the javascript implementation use the same algorithm for
generating random strings.

gui/scripts/syncthing/core/controllers/syncthingController.js

@@ -994,7 +994,7 @@ angular.module('syncthing.core')
         };
 
         $scope.setAPIKey = function (cfg) {
-            cfg.APIKey = randomString(30, 32);
+            cfg.APIKey = randomString(32);
         };
 
         $scope.showURPreview = function () {