This PR adds a new `billing_customers` table to hold the billing
customers.
Previously we were storing both the `stripe_customer_id` and
`stripe_subscription_id` in the `billable_subscriptions` table. However,
this creates problems when we need to correlate subscription events back
to the subscription record, as we don't know the user that the Stripe
event corresponds to.
By moving the `stripe_customer_id` to a separate table we can create the
Stripe customer earlier in the flow—before we create the Stripe Checkout
session—and associate that customer with a user. This way when we
receive events down the line we can use the Stripe customer ID to
correlate it back to the user.
We're doing some destructive actions to the `billing_subscriptions`
table, but this is fine, as we haven't started using them yet.
Release Notes:
- N/A
