Update to v2.8.6 without release

- Code refactoring
- Update the song database
- Fix a bug of the friend system which arose in the last version
- Fix a bug of duplicate calculating finishing states when players finish playing the chart in Link Play.

latest version/database/singles.json

@@ -1180,5 +1180,23 @@
 		],
 		"orig_price": 100,
 		"price": 100
+	},
+	{
+		"name": "mu",
+		"items": [
+			{
+				"type": "single",
+				"id": "mu",
+				"is_available": true
+			},
+			{
+				"type": "core",
+				"amount": 1,
+				"id": "core_generic",
+				"is_available": true
+			}
+		],
+		"orig_price": 100,
+		"price": 100
 	}
 ]