Rename interface

- Fix wrong inheritance in ArchiveModelManager
- Add license headers

osu.Game/Database/IModelManager.cs

@@ -1,20 +1,14 @@
-
+// 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;
 
 namespace osu.Game.Database
 {
     public interface IModelManager<TModel>
     {
-        /// <summary>
-        /// Fired when a new <see cref="TModel"/> becomes available in the database.
-        /// This is not guaranteed to run on the update thread.
-        /// </summary>
         event Action<TModel, bool> ItemAdded;
 
-        /// <summary>
-        /// Fired when a <see cref="TModel"/> is removed from the database.
-        /// This is not guaranteed to run on the update thread.
-        /// </summary>
         event Action<TModel> ItemRemoved;
     }
 }