Fix bugs in backend logic, database logic; Add features

[andychenggg]

1. update register/login UI.
2. add a feature: logout
3. update register/login logic.
4. fix a bug: Remove todoList from database: a user can exist without a todolist
5. update requirements.txt and db.sqlite
6. Add a delete button: for every todo-list
7. Add a delete button: for each item in todo-list