Logout function with Flask

[YuqianZhang]

As a front-end developer
I need set up the logout function
So that it will not be annoyed by time out issue

Assumptions:

• New route /logout which directs to the function logout() created
• Validating the login credentials with SqlAlchemy is supported

Acceptance Criteria:
When time expires,
then it closes the current session and returns the login interface for security