Django 2.3 Guestbook model-method

Encapsulate methods to model Guestbook, Greeting
- Create model Guestbook
- Implement method get_lastest_greeting, put_greeting_with_data in
model Guestbook

Demo url: http://upheld-quasar-641.appspot.com
- hãy định nghĩa CONSTANT cho 'default_guestbook' bên ngoài class
- nên thiết kế hàm get_default_name( ) cho class Guestbook