feat: create a sessions table to authenticate logged in users

[BaoNguyen09]

Describe your changes (Include screenshots if neccessary)

• Create a new table called sessions to track authenticated user who logged in
• Upgrade get_cookie() method to get_authenticated_user() that returns None if the username field in cookie isn't found in sessions table

Issue ticket number and link

Closes #19

Checklist before requesting a review

• I have performed a self-review of my code
• If it is a core feature, I have added thorough tests.
• I have commented my code, particularly in hard-to-understand areas
• New and existing unit tests pass locally with my changes