Add support for creating new PM conversations

osu.Game/Online/API/Requests/CreateNewPrivateMessageResponse.cs

@@ -0,0 +1,16 @@
+// Copyright (c) 2007-2018 ppy Pty Ltd <contact@ppy.sh>.
+// Licensed under the MIT Licence - https://raw.githubusercontent.com/ppy/osu/master/LICENCE
+
+using Newtonsoft.Json;
+using osu.Game.Online.Chat;
+
+namespace osu.Game.Online.API.Requests
+{
+    public class CreateNewPrivateMessageResponse
+    {
+        [JsonProperty("new_channel_id")]
+        public int ChannelID;
+
+        public Message Message;
+    }
+}