collab: Record cancellation reason on billing subscriptions (#22853)

This PR updates the `billing_subscriptions` in the database to record
the cancellation reason from Stripe.

We're primarily interested in this so we can check for subscriptions
that were canceled for being `past_due`.

Release Notes:

- N/A

crates/collab/migrations.sqlite/20221109000000_test_schema.sql

@@ -438,7 +438,8 @@ CREATE TABLE IF NOT EXISTS billing_subscriptions (
     billing_customer_id INTEGER NOT NULL REFERENCES billing_customers(id),
     stripe_subscription_id TEXT NOT NULL,
     stripe_subscription_status TEXT NOT NULL,
-    stripe_cancel_at TIMESTAMP
+    stripe_cancel_at TIMESTAMP,
+    stripe_cancellation_reason TEXT
 );
 
 CREATE INDEX "ix_billing_subscriptions_on_billing_customer_id" ON billing_subscriptions (billing_customer_id);