Add equatable support to IUser and RealmUser

Not sure this will stick, but let's add it for now to make testing
detach support work nicely.

osu.Game/Users/IUser.cs

@@ -1,14 +1,25 @@
 // 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 System;
 using osu.Game.Database;
 
+#nullable enable
+
 namespace osu.Game.Users
 {
-    public interface IUser : IHasOnlineID<int>
+    public interface IUser : IHasOnlineID<int>, IEquatable<IUser>
     {
         string Username { get; }
 
         bool IsBot { get; }
+
+        bool IEquatable<IUser>.Equals(IUser? other)
+        {
+            if (other == null)
+                return false;
+
+            return OnlineID == other.OnlineID && Username == other.Username;
+        }
     }
 }