feat(stripe): handle Stripe subscription events

[tianrunhe]

Description

When we increase the team subscription price from $6 to $8, it would be a new pricing (parabol-pro-800 vs. parabol-pro-600) and that means we need to migrate all customers from the old subscriptions to new ones. I plan to do the migration with a separate script that interacts with Stripe directly. The change we need from Parabol app side is to listen for the subscription changes and then update the stripeSubscriptionId field for the org accordingly.

Testing scenarios

1. Start a local listener stripe listen --forward-to localhost:3000/stripe
2. Upgrade an org to team tier and verify the stripeId and stripeSubscriptionId in Organization table match with the customerId and subscriptionId in Stripe
3. Go to the customer page in Stripe, cancel that subscription. Verify the stripeSubscriptionId in Organization table is removed.
4. Go back to the customer page in Stripe and manually create a new subscription. Verify the stripeSubscriptionId in the Organization table is updated with the subscriptionId from the new subscription.

Final checklist

• I checked the code review guidelines
• I have added Metrics Representative as reviewer(s) if my PR invovles metrics/data/analytics related changes
• I have performed a self-review of my code, the same way I'd do it for any other team member
• I have tested all cases I listed in the testing scenarios and I haven't found any issues or regressions
• Whenever I took a non-obvious choice I added a comment explaining why I did it this way
• I added the label Skip Maintainer Review Indicating the PR only requires reviewer review and can be merged right after it's approved if the PR introduces only minor changes, does not contain any architectural changes or does not introduce any new patterns and I think one review is sufficient'
• PR title is human readable and could be used in changelog

[Dschoordsch]

Apart from the function naming, it looks good and works