Support specific start date for subscription
- Make redirect flow
- Confirm it (creates mandate)
- Setup subscription ← this bit.
Subscription is created here: https://github.com/artfulrobot/uk.artfulrobot.civicrm.gocardless/blob/master/CRM/GoCardlessUtils.php#L222. See https://developer.gocardless.com/api-reference/#subscriptions-create-a-subscription for docs on
Stripe, IATs and Smartdebit CiviCRM extensions have an option to specify a
start date so we can use example code from them to collect the value.