collab: Limit customers to one free trial (#29232)

This PR makes it so customers can only subscribe to the trial once.

Release Notes:

- N/A

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

@@ -492,7 +492,8 @@ CREATE TABLE IF NOT EXISTS billing_customers (
     created_at TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP,
     user_id INTEGER NOT NULL REFERENCES users (id),
     has_overdue_invoices BOOLEAN NOT NULL DEFAULT FALSE,
-    stripe_customer_id TEXT NOT NULL
+    stripe_customer_id TEXT NOT NULL,
+    trial_started_at TIMESTAMP
 );
 
 CREATE UNIQUE INDEX "uix_billing_customers_on_user_id" ON billing_customers (user_id);