Code refactoring

- Code refactoring
- Fix a bug that the other player will not become the host of the room at once, when the player disconnect in link play.

> Maybe add many unknown bugs. XD
> The song database `arcsong.db` will not used in the future. You can use a tool in `tool` folder to import old data.

latest version/api/users.py

@@ -2,12 +2,12 @@ from flask import (
     Blueprint, request
 )
 
-from .api_code import code_get_msg, return_encode
+from .api_code import return_encode
 from .api_auth import role_required
 from core.user import UserRegister
 from core.error import ArcError, PostError
-from server.sql import Connect
-from server.sql import Sql
+from core.sql import Connect
+from core.sql import Sql
 import time
 import web.webscore
 import server.info