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
