Create Mandate Setup

Starts a direct debit setup with GoCardless. Send the customer's browser to the returned redirectUrl; GoCardless sends it back to the successRedirectUrl with redirect_flow_id appended, and the client completes the setup with PUT /me/billing/mandate, passing the session token it was given here. 409 when the caller already has an active or in-progress mandate; 400 when the caller has no email address or GoCardless refused.