collab: Add billing_subscriptions table (#15448)

This PR adds a new `billing_subscriptions` table to the database, as
well as some accompanying models/queries.

In this table we store a minimal amount of data from Stripe:

- The Stripe customer ID
- The Stripe subscription ID
- The status of the Stripe subscription

This should be enough for interactions with the Stripe API (e.g., to
[create a customer portal
session](https://docs.stripe.com/api/customer_portal/sessions/create)),
as well as determine whether a subscription is active (based on the
`status`).

Release Notes:

- N/A

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

@@ -416,3 +416,16 @@ CREATE TABLE dev_server_projects (
     dev_server_id INTEGER NOT NULL REFERENCES dev_servers(id),
     paths TEXT NOT NULL
 );
+
+CREATE TABLE IF NOT EXISTS billing_subscriptions (
+    id INTEGER PRIMARY KEY AUTOINCREMENT,
+    created_at TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP,
+    user_id INTEGER NOT NULL REFERENCES users(id),
+    stripe_customer_id TEXT NOT NULL,
+    stripe_subscription_id TEXT NOT NULL,
+    stripe_subscription_status TEXT NOT NULL
+);
+
+CREATE INDEX "ix_billing_subscriptions_on_user_id" ON billing_subscriptions (user_id);
+CREATE INDEX "ix_billing_subscriptions_on_stripe_customer_id" ON billing_subscriptions (stripe_customer_id);
+CREATE UNIQUE INDEX "uix_billing_subscriptions_on_stripe_subscription_id" ON billing_subscriptions (stripe_subscription_id);