This PR updates the `billing_subscriptions` table with some new columns

- `kind` - The kind of the description (used to denote Zed Pro vs
existing)
- `stripe_current_period_start` - The Stripe timestamp of when the
subscriptions current period starts
- `stripe_current_period_end` - The Stripe timestamp of when the
subscriptions current period ends

Release Notes:

- N/A

Co-authored-by: Mikayla <mikayla@zed.dev>