This PR adds a new `POST /billing/subscriptions/migrate` endpoint for
migrating users to the new billing system.

When called with a GitHub user ID this endpoint will:

1. Find the active billing subscription for this user (if they have one)
2. Cancel the subscription and send a final invoice
3. Ensure the user is in the `new-billing` and `assistant2` feature
flags

Release Notes:

- N/A