Feat/integrate with stripe webhook

[dangvuthang]

Changelog

Backend

• Add route for Stripe to notify checkout.session.completed POST /webhook
• Add route to check for whether logged-in user has purchased the current visited room POST /api/transactions/checkIfPaid

Frontend

• Conditional render Join room or Buy now button
• Notify user of successfully purchase once Stripe navigate user back to /my-rooms

Note: Since Stripe webhook require a deploy app to notify event. This pr will be reverted if there are any found errors

Closes #101

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