enable push notifications to use TPNS by default (#9774)

config/default.json

@@ -190,8 +190,8 @@
         "SMTPPort": "2500",
         "ConnectionSecurity": "",
         "InviteSalt": "",
-        "SendPushNotifications": false,
-        "PushNotificationServer": "",
+        "SendPushNotifications": true,
+        "PushNotificationServer": "https://push-test.mattermost.com",
         "PushNotificationContents": "generic",
         "EnableEmailBatching": false,
         "EmailBatchingBufferSize": 256,