DB 변경(sqlite3 -> mysql)
anonymous_user 를 AnonymousUserMixin(flask.ext.login) 으로 변경
파일 크기가 저장되는 필드의 타입을 Int32 에서 BigInt 로 변경
파일 경로 String length 정해지지 않아 발생하는 문제 해결
가입된 유저임을 확인할 때 current_user object 의 is_authenticated 함수를 호출하여 확인하도록 수정