Allow retrieving count of available stable imports

osu.Game/Collections/CollectionManager.cs

@@ -111,6 +111,18 @@ namespace osu.Game.Collections
 
         public Action<Notification> PostNotification { protected get; set; }
 
+        public Task<int> GetAvailableCount(StableStorage stableStorage)
+        {
+            if (!stableStorage.Exists(database_name))
+                return Task.FromResult(0);
+
+            return Task.Run(() =>
+            {
+                using (var stream = stableStorage.GetStream(database_name))
+                    return readCollections(stream).Count;
+            });
+        }
+
         /// <summary>
         /// This is a temporary method and will likely be replaced by a full-fledged (and more correctly placed) migration process in the future.
         /// </summary>