Rename User to APIUser and move to correct namespace

osu.Game/Users/IUser.cs

@@ -0,0 +1,12 @@
+// Copyright (c) ppy Pty Ltd <contact@ppy.sh>. Licensed under the MIT Licence.
+// See the LICENCE file in the repository root for full licence text.
+
+using osu.Game.Database;
+
+namespace osu.Game.Users
+{
+    public interface IUser : IHasOnlineID<int>
+    {
+        string Username { get; set; }
+    }
+}