This PR adds a new `POST /billing/subscriptions/manage` endpoint that
can be used to manage a billing subscription.
The endpoint accepts a `github_user_id` to identify the user, as well as
an optional `subscription_id` for managing a specific subscription. If
`subscription_id` is not provided, it try and use the active
subscription, if there is only one.
Right now the endpoint only supports cancelling an active subscription.
This is done by passing `"intent": "cancel"` in the request body.
The endpoint will return the URL to a Stripe customer portal session,
which the caller can redirect the user to.
Here's an example of how to call it:
```sh
curl -X POST "http://localhost:8080/billing/subscriptions/manage" \
-H "Authorization: <ADMIN_TOKEN>" \
-H "Content-Type: application/json" \
-d '{"github_user_id": 12345, "intent": "cancel"}'
```
Release Notes:
- N/A