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
Fix comment

- create constants
- change instance method to class method
Remove unnecessary import
Merge pull request #4 from nhathv/feature/model-method

Merged pull request #4 Django 2.3 Guestbook model-method